FROM  THE  LIBRARY  OF 
WILLIAM  A.  SETCHELL,i864-i943 

PROFESSOR  OF  BOTANY 


BULLETIN 


ILLINOIS  STATE  LABORATORY 


NATURAL  HISTORY 

URBANA,  ILLINOIS,  U.  S.  A. 
VOL.  VIII.  MAY,  1908  ARTICLE  I. 


THE  PLANKTON  OF  THE  ILLINOIS  RIVER,  1894-1899,  WITH 
INTRODUCTORY  NOTES  UPON  THE  HYDROGRAPHY  OF  THE  ILLINOIS 
RIVER  AND  ITS  BASIN.  PART  II.  CONSTITUENT  ORGANISMS  AND 
THEIR  SEASONAL  DISTRIBUTION. 


BY 

C.  A.  KOFOID,  PH.D. 


<  -2- 

B1OLOSY 
LIBRAIY 


BULLETIN 


ILLINOIS  STATE  LABORATORY 


NATURAL  HISTORY 

URBANA,  ILLINOIS,  U.  S.  A. 
VOL.  VIII.  ARTICLE  I. 


THE  PLANKTON  OF  f  THE  ILLINOIS  RIVER,  1894-1899,  WITH 
INTRODUCTORY  NOTES  UPON  THE  HYDROGRAPHY  OF  THE  ILLINOIS 
RIVER  AXD  ITS  BASIN.  PART  II.  CONSTITUENT  ORGANISMS  AND 
THEIR  SEASONAL  DISTRIBUTION. 


BY 

C.  A.  KOFOID,  PH.D. 


THE   ILLINOIS  PRINTING  COMPANY 
DANVILLE,  ILLINOIS 


BIOLOGY  LIBRARY 


CONTENTS. 


PAGE. 

Introduction i_i 7 

Distribution  of  Collections  by  Months  (Table) 4 

Methods 5 

Acknowledgments 7 

Definitions 7 

The  Composition  of  the  Plankton 10 

Comparison  of  Fresh-water  and  Marine  Plankton 12 

Organisms  of  the  Plankton 13 

Constituent  Groups  of  the  Annual  Plankton  of  the  Illinois  River  (Table)  16 
Discussion  of  the  Statistical  Data  of  the  Species  composing  the  Plankton 

of  the  Illinois  River  in  1894-1899 18-290 

Cryptogamia ; 18-62 

Bacteriaceae 18 

Schizophyceae 19 

Discussion  of  Species 20 

Chlorophyceae 22 

Discussion  of  Species 24 

Bacillariaceae 34 

Factors  controlling  Diatom  Production 35-43 

Diagram  showing  the  seasonal  distribution  of  diatoms,  total 
plankton,  nitrates,  and  thermograph  and  hydrograph  of  Illi- 
nois River  at  Havana  for  1898 37 

Discussion  of  Species 43 

Conjugatae 59 

Discussion  of  Species 60 

Phanerogamia .  62 

Protozoa 62 

Mastigophora 63 

Location  and  Amplitude  of  Pulses  of  Chlorophyll-bearing  Organ- 
isms in  the  Illinois  River  (Table) 64 

Discussion  of  Species 68 

Rhizopoda 92 

Discussion  of  Species 95 

Heliozoa 113 

Discussion  of  Species 113 

Sporozoa 114 

Ciliata 115 

Discussion  of  Species 118 

Suctoria • 131 

Discussion  of  Species 131 

Porifera..  132 


SZ246526 


PAGE 

Coelenterata 132 

Platyhelminthes 133 

Turbellaria 133 

Trematoda 134 

Cestoda 135 

Nemertini 135 

Nematelminthes 135 

Nematoda 135 

Acanthocephala 136 

Annulata 136 

Oligochaeta 136 

Naididag 137 

^Eolosomatidag 137 

Rotifera 138 

Rhizota 139 

Discussion  of  Species 140 

Bdelloida 141 

Discussion  of  Species 141 

Ploima 144 

Pulses  of  Ploima  (Table) 146 

Discussion  of  Species 147 

Tabular  statistical  data : — 

Pulses  of  Anurcea  cochlearis ISO,  153 

Pulses  of  A nurasa  hypelasma 155 

Pulses  of  Asplanchna  brightwellii 156 

Evidence  of  polycyclic  character  of  seasonal  distribution  of  above .  158 

Evidence  of  same  for  Asplanchna  priodonta 161 

Pulses  of  Brachionus  angularis 165 

Collection  data  for  Brachionus  bakeri  and  varieties 169,  170 

Pulses  of  Brachionus  budapestinensis 176 

Pulses  of  Brachionus  militaris 179 

Pulses  of  Brachionus  mollis 181 

Brachionus  pala  and  varieties.     Average  number  per  collection .  182 

Pulses  of  Brachionus  pala  and  varieties 184 

Brachionus  pala,  type  form.     Sexual  cycle 188 

Seasonal  distribution  of  B.  pala  and  its  var.  amphiceros 189 

Seasonal  limits  of  B.  pala  var.  dorcas 191 

Pulses  of  Brachionus  urceolaris 193 

Brachionus  urceolaris  and  varieties  and  B.  variabilis.     Average 

number  per  collection 196 

Pulses  of  Polyarthra  platyptera 204 

Pulses  of  Synchceta  pectinata 211 

Pulses  of  Synch&ta  stylata 213 

Pulses  of  Triarthra  longiseta 218 

Scirtopoda 219 

Seasonal  fluctuations  of  Pedalion  mirum  (Table) 220 

Gastrotricha .  .  221 


PAGE 

Entomostraca 221 

Branchiopoda 222 

Cladocera 223 

Cladocera  and  Hydrographic  Fluctuations  (Table) 223 

Discussion  of  Species 225 

Tabular  statistical  data: — 

Bosmina  and  hydrographic  fluctuations .  r- 228 

Seasonal  distribution  of  Chydorus.     Average  number  per  m.3. .  .  235 

Effect  of  temperature  on  numbers  of  Chydorus 238 

A  typical  pulse  of  Daphnia  cucullata 242 

Diaphanosoma  population,  with  data  of  temperature  and  river 

level 248 

Moina  and  hydrographic  changes 254 

Ostracoda 257 

Discussion  of  Species 258' 

Copepoda 258 

Copepoda  and  Hydrographic  Changes  (Table) 260 

Discussion  of  Species 261 

Cyclops  viridis  var.  insectus  and  Hydrographic  Changes  (Table).  .    274,  275 

Relative  numbers  of  adult,  young,  and  larval  Cyclopidae  (Table). .  278 

Amphipoda 283 

Arachnida 283 

Acarina 283 

Tardigrada 284 

Hexapoda 284 

Ephemerida 285 

Hemiptera 285 

Diptera 285 

Mollusca 287 

Gastropoda ' 287 

Lamellibranchiata 287 

Bryozoa 288 

Discussion  of  Species 288 

The  Periodicity  in  the  Multiplication  of  .the  Organisms  of  the  Plankton. . .   291-311 

Comparison  of  Plankton  Pulses  and  Lunar  Cycle  (Table) 296-299 

Gaseous  Contents  of  Pond  Water  (Table) 306 

Relation  of  Pulses  of  Chlorophyll-bearing  Organisms  to  Lunar  Cycle 

(Table) 308 

General  Considerations  on  Seasonal  Changes 312 

Lake  versus  River  Plankton 312 

Organisms  per  Cubic  Meter  in  Plankton  of  Illinois  River  in  1898  (Table) . .   314-340 

Bibliography 341-354 

Explanation  of  Plates 355 

Diagrams  of  Seasonal  Distribution 356—360 

Errata  and  Addenda.  .  361 


ARTICLE  I. — Plankton  Studies.  V.1  The  Plankton  of  the  Illi- 
nois River,  1894-1899.  Part  II.  Constituent  Organisms  and  their 
Seasonal  Distribution.  BY  C.  A.  KOFOID. 

INTRODUCTION. 

This  paper  gives  the  results  of  a  statistical  study  of  a  series  of 
quantitative  plankton  collections  made  in  the  channel  of  the  Illinois 
River  near  Havana,  111.,  at  the  Illinois  Biological  Station,  in  1894- 
1899.  The  environmental  conditions  and  the  volumetric  results  of 
this  investigation  have  been  given  in  Part  I.  (Kofoid,  '03),  published 
in  Volume  VI.  of  this  Bulletin. 

Of  the  235  collections  made  in  channel  waters  and  used  in  the 
quantitative  study,  only  182  were  subjected  to  numerical  and 
qualitative  analysis.  The  omitted  collections  were  intercalated  at 
brief  intervals  of  one  to  several  days  between  those  enumerated, 
principally  in  the  summer  of  1895  and  during  the  winter  flood  of 
1896  and  the  summer  of  the  same  year.  The  collections  chosen  for 
this  study,  whenever  possible,  represent  a  weekly  interval,  and  a 
full  list  of  all  collections,  with  environmental  data,  may  be  found  in 
Table  III.  of  Part  I.  The  chronological  distribution  of  the  collec- 
tions studied  by  the  statistical  method  is  given  in  the  table  on 
the  following  page. 

The  work  of  enumeration  and  the  primary  tabulation  was  com- 
pleted at  Urbana  December  31,  1900,  when  my  formal  connection 
with  the  State  Laboratory  ceased.  The  manuscript  has  been 

1  The  four  preceding  numbers  of  this  series,  all  by  the  present  writer,  have 
been  published  as  articles  of  the  Bulletin  of  the  Illinois  State  Laboratory  of 
Natural  History,  as  follows: — 

Article  I.,  Vol.  V. — Plankton  Studies.  I.  Methods  and  Apparatus  in  Use  in 
Plankton  Investigations  at  the  Biological  Experiment  Station  of  the  University  of 
Illinois. 

Article  V.,  Vol.  V. — Plankton  Studies.  II.  On  Pleodomna  illinoisensis ,  a  New 
Species  from  the  Plankton  of  the  Illinois  River. 

Article  IX.,  Vol.  V. — Plankton  Studies.  III.  On  Platydorina,  a  New  Genus 
of  the  Family  Volvocidce,  from  the  Plankton  of  the  Illinois  River. 

Article  II.,  Vol.  VI. — Plankton  Studies.  IV.  The  Plankton  of  the  Illinois 
River,  1894-1899,  with  Introductory  Notes  upon  the  Hydrography  of  the  Illinois 
River  and  its  Basin.  Part  I.  Quantitative  Investigations  and  General  Results. 


prepared  at  Berkeley,  being  completed  in  May,  1904,  after  my 
connection  with  the  University  of  California  was  begun.  My 
separation  from  the  collections  and  the  library  of  the  State  Labora- 
tory has  rendered  impossible  some  verifications,  comparisons  of 
specimens  with  more  recent  literature,  especially  among  the  algae, 

DISTRIBUTION  OP  COLLECTIONS  BY  MONTHS. 


'94. 

•91 

'96. 

'97. 

'98. 

'99. 

I  

4 

3 

5 

II  

1 

4 

2 

4 

4 

Ill  

5 

1 

5 

4 

IV  

2 

4 

1 

4 

V  

4 

1 

5 

VI  

2 

1 

5 

1 

4 

VII  

2 

4 

5 

3 

4 

VIII  

1 

5 

6 

4 

5 

IX..  

2 

4 

2 

4 

4 

X  

1 

5 

1 

4 

4 

XI  

1 

4 

1 

5 

5 

XII  

1 

5 

2 

4 

5 

Total  

10 

31 

43 

30 

52 

13 

some    desirable   amplifications   from   omitted  intermediate  collec- 
tions, and  the  elimination  of  a  few  minor  errors  in  the  statistics. 

It  should  be  understood  that  the  data  of  this  paper  are  derived 
from  channel  collections,  and  the  conclusions  apply  only  to  that 
region.  Conditions  of  plankton  development  in  the  adjacent  back- 
waters, as  shown  in  Part  I.,  differ  greatly  in  volumetric  character 
and  seasonal  distribution.  The  composition  of  the  plankton  and 
the  seasonal  distribution  of  its  constituent  organisms  also  exhibit 
there  many  points  of  difference  from  those  here  described  for  channel 
waters. 


METHODS. 

The  collections  were  preserved  in  bottles  of  uniform  capacity 
(60  cm.3),  in  alcohol-formalin  mixture  (2  per  cent,  formalin  in  70 
per  cent,  alcohol),  and  after  measurement  by  the  centrifuge  were 
released  from  the  compressed  condition  in  the  measuring  tubes  and 
returned  to  the  containers. 

The  counting  was  done  by  a  modified  Sedgwick-Rafter  method 
(see  Kofoid,  '97),  in  which  1  cm.3  of  a  suitably  diluted  plankton  is 
distributed  evenly  in  a  cell  20  X  50  mm.  The  plankton  was  diluted 
or  condensed  (from  60  cm.3  of  fluid)  according  to  the  quantity  of 
plankton  and  the  amount  and  nature  of  the  silt.  Larger  organisms 
such  as  the  Entomostraca  were  counted  in  the  whole  catch,  or  in 
larger  collections  in  Yio  to  1/50  of  the  total  catch ;  and  the  smaller 
organisms  in  ll^  to  V^o-  The  filter-paper  catches  which  supple- 
mented those  of  the  plankton  net  from  August  3,  1896,  to  the  end 
of  the  series,  March  28,  1899,  were  often  subjected  to  considerable 
dilution  on  account  of  the  great  amount  of  fine  silt  in  the  collections, 
from  Vio  "to  Yioo  being  the  limits  of  dilution  as  a  rule. 

The  even  distribution  of  the  organisms  in  the  Rafter  cell  was 
secured  by  shaking  the  collection  in  a  mixing  cylinder  gently  till 
the  sediment  was  thoroughly  distributed,  and  taking  the  sample 
immediately  with  a  long  1  cm.3  pipette,  inserted  to  the  bottom  of 
the  jar  and  raised  to  the  surface  during  the  filling  process,  and  by 
discharging  the  contents  immediately  into  the  cell  at  one  corner, 
the  cover  having  been  previously  displaced  at  a  slight  obliquity  to 
admit  the  end  of  the  pipette.  With  the  filling  of  the  cell  the  cover 
automatically  moves  into  place,  and  practice  soon  enables  one  to 
fill  the  cell  without  inclusion  of  air  bubbles.  With  the  exception 
of  the  heavier  rhizopods,  all  of  the  organisms  are  as  a  rule  very 
evenly  distributed  by  this  method. 

The  identification  and  enumeration  of  the  contents  of  the  cell 
were  carried  on  with  the  help  of  a  mechanical  stage  and  a  f  Bausch 
&  Lomb  objective,  with  a  Zeiss  C  for  higher  magnification  when 
needed  for  the  detection  of  fine  details  or  for  counting  the  smaller 
organisms  in  the  filter-paper  catches. 

After  considerable  experimenting,  the  following  method  was 
established  in  the  work  of  enumeration.  Four  sheets,  each  with 
numbers  1  to  76  at  the  left,  were  fastened  temporarily  to  accom- 


panying  key  sheets,  each  number  on  each  sheet  standing  for  one  of 
the  more  common  species.  One  sheet  was  assigned  to  algae, 
diatoms,  and  miscellaneous  organisms;  and  one  each  to  Protozoa, 
Rotifer  a,  and  Entomostraca.  As  the  plankton  sample  was  examined 
under  the  microscope  the  identifications  were  called  off,  and  entered 
on  the  sheets  by  a  clerical  assistant.  Six  of  the  most  abundant 
species  were  recorded  by  the  observer  himself  on  six  tallying 
machines  registering  1,000,  and  conveniently  arranged  in  a  box  at 
his  right.  By  adjusting  the  springs  to  give  different  sounds  when 
registry  was  made,  and  by  modifying  the  surfaces  pressed  by  the 
fingers  so  as  to  differentiate  the  several  machines  without  looking 
at  them,  it  wras  possible  to  use  these  without  raising  the  eye  from 
the  microscope,  and  thus  to  avoid  the  fatigue  arising  from  the 
repeated  muscular  readjustment  of  the  eyes  necessary  when  the 
observer  makes  his  own  entries  in  a  written  record.  Common 
species  not  recorded  by  the  tallying  machines  were  generally  abbre- 
viated or  designated  by  easily-called  tokens.  When  once  fairly 
familiar  with  the  species  it  was  possible  by  means  of  these  labor- 
saving  devices  to  make  identifications  and  enumerations  of  several 
heavy  planktons  per  day. 

By  a  number  of  tests  I  found  that  when  the  enumerations  of  a 
species  in  a  given  collection  reached  1,000,  little  was  gained  by 
carrying  it  to  higher  numbers.  A  limit  of  error  of  ±5  per  cent. 
can  be  thus  obtained  if  the  species  in  question  is  distributed  evenly 
in  the  cell  and  all  precautions  are  observed  to  secure  accuracy. 
Enumerations  were  often  carried  beyond  this  point,  but  rarely  beyond 
3,000.  The  accessions  numbers  of  the  collections  from  our  catalog 
of  collections  served  to  designate  each  sheet  of  data  and  all  note 
slips  bearing  on  the  collection  or  its  constituent  organisms.  When 
the  enumeration  was  completed,  the  factors  of  collection,  dilution, 
and  enumeration  were  entered  on  the  sheets,  and  the  number  of 
individuals  of  all  species  represented  was  computed  and  carried  to 
the  right  of  the  sheet.  The  totals  of  the  various  groups — for  ex- 
ample, diatoms  or  Cladocera — were  then  added  up  and  entered  on 
the  sheets  in  differential  colors.  By  the  use  of  the  key  sheets  the 
number  perm.3  of  water  of  any  given  species  could  be  quickly  ascer- 
tained. Species  not  in  the  key  were  entered  by  name  on  the  sheets. 

When  the  enumeration  of  all  collections  was  completed,  the 
numbers  per  m.3  giving  the  seasonal  distribution  of  the  various 


7 

species  and  groups  through  the  collections  of  1894-1899  were  drawn 
up  on  uniform  folio  sheets,  and  the  annual  totals  and  averages  com- 
puted therefrom.  With  the  data  in  these  forms  it  is  possible  to 
turn  at  once  to  the  statistics  of  the  plankton  of  a  given  day,  or  to 
the  seasonal  distribution  of  any  desired  species. 

ACKNOWLEDGMENTS. 

I  am  indebted  to  Prof.  S.  A.  Forbes,  Messrs.  E.  B.  Forbes,  F.  W. 
Schacht,  and  R.  W.  Sharpe  for  many  suggestions  concerning  the 
Entomostraca;  to  Prof.  Frank  Smith  for  assistance  with  the  Oligo- 
ch&ta  of  the  plankton ;  and  to  Mr.  A.  Hempel  for  my  introduction  to 
the  Rotifera.  The  identification  of  the  cosmopolitan  species  of  the 
fresh-water  plankton  of  the  Illinois  River  was  greatly  facilitated  by 
the  most  excellent  library  of  the  Illinois  State  Laboratory  of  Natu- 
ral History,  the  accumulation  of  many  years'  careful  selection  by 
its  director,  Prof.  S.  A.  Forbes.  The  literature  of  fresh- water  fauna, 
and  to  a  large  extent  of  its  flora  also,  is  very  fully  represented  therein. 
The  excellent  Laboratory  collection  of  identified  Entomostraca 
from  European  specialists  was  also  of  great  service. 

I  am  indebted  to  Mr.  R.  E.  Richardson  for  valuable  services  as 
clerical  assistant,  and  for  substantial  help  in  organizing  the  great 
mass  of  data  resulting  from  the  enumerations. 

'  Except  as  noted  in  the  discussion  in  subsequent  pages,  I  hold 
myself  responsible  for  all  of  the  identifications  of  the  species  re- 
corded. The  enumeration  is  also  all  my  own  work,  with  the  excep- 
tion of  that  of  the  nauplii,  of  two  species  oi^ttifflugia,  and  of  Pedi- 
astrum  in  about  one  third  of  the  collections,  in  which  I  had  the 
assistance  of  Mr.  R.  J.  DeMotte,  and  that  of  the  commoner  Rotifera 
in  a  few  of  the  collections,  which  were  counted  by  Mr.  Richardson. 

DEFINITIONS. 

The  term  "plankton"  was  used  by  Hensen  ('87)  to  designate 
"Alles  was  im  Wasser  treibt."  It  was  applied  by  him  only  to  that 
assemblage  of  marine  organisms  which  float  passively  in  the  open 
sea,  without  active  recourse  to  shore  or  bottom,  and  unable  by  their 
own  efforts  materially  to  change  their  location.  The  term  has 
since  been  extended  also  to  assemblages  of  organisms  in  fresh  water 
which  bear  a  similar  relation  to  open  water.  This  fresh-water 


plankton  has  been  designated  in  turn  "  limnoplankton  "  by  Haeckel 
('90),  a  word  which  in  a  restricted  sense  is  retained  for  the  plankton 
of  lakes,  while  that  of  rivers  has  been  distinguished  by  Zacharias 
('98a)  as  "potamoplankton,"  and  that  of  ponds  ('98)  as  "heleo- 
plankton."  These  distinctions  are  based  upon  the  nature  of  the 
environing  body  of  water,  and  the  terms  are  convenient,  though 
the  separation  of  these  types  everywhere  in  nature  is  difficult,  if  not 
impossible.  Owing  to  the  smaller  size  of  fresh-water  basins  as 
compared  with  those  of  marine  character,  the  shore  and  bottom  be- 
come more  important  as  factors  in  the  environment  of  the  plankton. 
Within  the  fresh-water  environment  we  also  find  degrees  of  impor- 
tance of  the  shore  and  bottom  which  in  ascending  scale  dominate 
in  the  lake,  river,  pond,  and  marsh.  Although  each  of  these  repre- 
sents distinct  conceptions,  in  nature  we  find  them  imperceptibly 
intergrading,  and  neither  these  conceptions,  geographical  nomen- 
clature, nor  local  parlance  give  us  any  final  criterion  which  will 
enable  us  to  use  the  terms  with  the  precision  which  a  scientific 
terminology  would  demand.  The  distinctions  between  these  forms 
of  fresh-water  plankton  must  lie  in  the  plankton  itself,  if  anywhere. 
As  I  shall  attempt  to  show  later,  these  distinctions,  though  appar- 
ent, in  some  cases  at  least,  are  nevertheless  of  minor  importance, 
and  depend  very  largely  upon  the  relative  predominance  of  the 
adventitious  littoral  fauna  and  flora  rather  than  upon  distinctive 
assemblages  of  eulimnetic  species.  The  striking  similarity  of  this 
eulimnetic  plankton  in  all  these  types  of  environment  and  in  widely 
separated  continents  is  a  biological  phenomenon  of  far  more  sig- 
nificance than  these  minor  differences.  These  distinctions  between 
the  different  types  of  fresh-water  plankton  are  thus  more  a  matter 
of  terminology  than  of  biological  import. 

Among  the  organisms  found  in  open  water  there  are  varying 
degrees  of  dependence  upon  the  shore  and  bottom.  Some,  as 
Cyclops  and  many  of  the  lower  algas,  have  life  cycles  in  which  no 
encysted  or  quiescent  resting  stage  has  been  found,  and  actively  or 
passively  their  whole  existence  is  passed  in  the  open  water.  They 
are  at  all  times  components  of  the  plankton;  that  is,  are  continuous 
planktonts.  Others,  as  Dinobryon,  many  of  the  Rotifera  and  Cladoc- 
era,  and,  in  fact,  the  greater  part  of  the  eulimnetic  organisms,  have 
an  encysted  stage  which  as  a  winter  egg  or  a  cyst  descends  to  the 
bottom  and  remains  there  for  a  season.  Such  organisms  only 


periodically,  wholly  or  in  part,  leave  the  open  water  for  a  littoral 
or  benthal  existence.  They  are  periodic  planktonts.  Some  organ- 
isms, such  as  many  of  the  rhizopods  and  diatoms  and  Hydra,  appear 
in  the  plankton  under  certain  conditions  of  temperature  and  food. 
They  temporarily  adopt  the  limnetic  mode  of  life  as  a  result  either 
of  a  change  in  their  specific  gravity  due  to  internal  changes,  such  as 
an  increase  of  the  gaseous  or  fatty  contents  of  their  protoplasm,  or 
to  changes  in  the  buoyancy  of  the  water  due  to  changes  in 
temperature  or  in  substances  in  solution  in  the  water,  or  because 
of  the  abundance  of  food  in  the  open-  water.  They  become 
under  these  conditions  actively  adventitious  planktonts.  Still  other 
organisms  are  released  from  their  usual  contact  with  or  attach- 
ment to  the  substratum,  or  from  their  association  with  debris 
or  vegetation  of  shore  or  bottom,  by  movements  or  disturbances  in 
the  water,  and  are  swept  into  the  open  water  only  to  return  again 
to  their  customary  habitat  when  conditions  favor.  Practically  all 
of  the  smaller  organisms  inhabiting  the  shore  and  bottom  and  the 
debris  and  vegetation  found  thereon  are  liable  thus  to  enter  the 
open  water,  and  to  be  found  in  forced  and  temporary  association 
with  the  eulimnetic  fauna  and  flora.  They  are  passively  adventi- 
tious planktonts. 

Another  class  of  organisms  which  occur  in  the  plankton  are  those 
which  either  as  internal  or  external  parasites  find  in  plankton  organ- 
isms either  a  host  or  a  substratum  for  attachment.  These  are  in  a 
certain  sense  passive  planktonts,  and  they  may  be  distinguished  from 
other  passive  planktonts  as  attached  or  parasitic  planktonts.  Sharp 
lines  between  these  various  classes  of  organisms  found  in  open  water 
can  not  be  drawn  upon  distinctions  based  upon  their  degree  of 
dependence  upon  the  bottom  and  shore.  An  equally  vague  line 
separates  the  organisms  of  the  plankton  from  those  more  active 
forms  which  by  virtue  of  their  powers  of  locomotion  are  to  a  con- 
siderable degree  independent  of  waves  and  current,  and  are  able 
freely  to  maintain  their  position  in  their  preferred  habitat.  Among 
the  organisms  commonly  included  in  the  plankton,  the  flagellates, 
rotifers,  and  Entomostraca  exhibit  some  degree  of  activity,  such  as 
is  seen  in  their  limited  vertical  migrations,  while  larger  organisms, 
such  as  Leptodora  hyalina  and  the  larvae  of  Corethra,  are  capable  of 
movement  sufficient  to  give  them  considerable  independence  in  the 
matter  of  their  position  in  the  water.  We  thus  find  degrees  of  inde- 


10 

pendence  which  approach  closely  that  found  in  young  fish  and  the 
large  insect  larvae — organisms  not  always  regarded  as  planktonts. 

The  plankton  is  thus  a  composite  assemblage  of  organisms  whose 
association  depends  in  varying  degrees  upon  their  relation  to  their 
common  habitat,  the  open  water.  In  actual  practice,  all  the  organ- 
isms found  in  the  open  water  are  regarded  as  within  the  scope  of 
plankton  investigations,  and  justly  so,  for  by  virtue  of  their  pres- 
ence they  become  more  or  less  involved  in  the  complex  interrela- 
tions which  pertain  to  the  flux  of  matter,  the  succession  of  species, 
and  the  food  relations  which  exist  through  the  changing  seasons  in 
the  aquatic  environment. 

In  our  own  investigations  it  has  been  our  purpose  to  include  all 
the  organisms  found  in  our  collections ;  that  is,  all  which  our  meth- 
ods of  examination  give  us  a  sufficient  means  of  investigating. 
Naturally,  the  bacteria  are  to  large  extent  excluded  from  our  con- 
sideration, though  they  properly  belong  to  the  plankton,  and  in  the 
processes  of  nitrification  and  denitrification  play  an  exceedingly  im- 
portant part  in  the  economy  of  aquatic  life. 

THE    COMPOSITION    OF    THE    PLANKTON. 

The  composite  character  of  the  plankton  is  especially  marked  in 
streams , — as,  for  example ,  in  the  Illinois  River, — owing  to  the  mingling 
of  organisms  from  a  great  variety  of  tributary  sources — backwaters, 
lakes,  ponds,  pools,  marshes,  swamps,  brooks,  rivers,  canals,  sewers, 
drains,  and  industrial  wastes.  Few  lakes  possess  so  varied  a  supply, 
and  in  none  can  the  proportional  effect  of  these  contributions  exceed 
that  of  the  stream.  Added  to  this  contributed  assemblage,  and  in 
some  seasons  predominating  over  it,  is  the  indigenous  or  autono- 
mous plankton  of  the  stream  itself. 

The  component  organisms  of  the  plankton  of  the  Illinois  River 
number  528  forms,  including  only  those  which  have  been  identified 
from  collections  made  in  the  main  stream  and  including  both 
species  and  well-defined  forms  or  varieties.  Species  found  thus  far 
only  in  the  backwaters  are  not  included,  though  there  is  little  doubt 
that  they  occur  also  in  the  main  stream.  No  effort  has  been  made 
to  build  up  merely  a  long  list  of  species,  but  only  to  identify,  so  far 
as  possible,  the  common  and  recurring  forms.  Neither  has  any 
attempt  been  made  to  establish  new  species  or  revise  those  already 


11 

described,  though  a  magnificent  opportunity  awaits  the  naturalist 
who  has  the  fortitude  to  analyze  the  exceedingly  variable  forms 
which  compose  the  plankton,  and  to  determine  by  modern  methods 
which  of  these  variants  are  entitled  to  specific  rank.  It  has  seemed 
to  the  writer  that  the  only  satisfactory  basis  upon  which  species, 
and  pre-eminently  those  of  the  fresh-water  plankton^  can  rest,  lies  in 
a  careful  determination  of  the  limits  of  seasonal  and  local  variation 
within  the  area  of  distribution.  This  means  breeding  under  con- 
trol, and  the  study  of  variation  by  modern  statistical  methods. 
Both  of  these  lines  of  inquiry  lie  beyond  the  purpose  of  the  present 
paper,  and  plainly  beyond  the  possibilities  of  accomplishment  by 
any  one  investigator,  when  the  great  number  of  species  and  the  pres- 
ent state  of  the  literature  of  the  subject  is  considered.  It  is  becom- 
ing constantly  more  evident  that  the  species  of  the  plankton  are  in 
the  main  cosmopolites,  and  the  world  literature  of  the  subject  must 
be  taken  into  consideration  in  any  thorough  attempt  to  handle  the 
systematic  side  of  the  subject.  During  the  progress  of  this  work, 
which  was  begun  in  1894,  every  effort  was  made  to  secure  all  perti- 
nent literature  bearing  on  the  genera  of  plants  and  animals  repre- 
sented in  the  plankton,  and  so  far  as  possible  in  the  enumeration  of 
the  collections  the  individuals  were  referred  to  "species"  already 
described,  or,  in  default  of  this,  recorded  as  "unidentified."  In 
some  groups — notably  the  desmids,  diatoms,  and  unicellular 
algas— it  was  not  possible  under  the  conditions  of  plankton  enumer- 
ation to  apply  to  all  the  individuals  enumerated  the  fine  distinc- 
tions which  specialists  in  these  groups  have  made.  They  have  been 
thrown  under  certain  of  the  better-defined  species,  which  thus  stand 
in  our  records  as  representatives  of  closely  related  variants  as  well 
as  of  the  types  of  the  species  named.  Examples  of  this  appear  in 
Closterium,  where  two  species  only  were  listed.  Probably  a  num- 
ber of  so-called  species  among  the  scores  described  in  this  genus 
will  be  found  among  the  individuals  in  our  plankton  here  referred 
to  the  two  species  C.  acerosum  and  C.  lunula.  So,  also,  in  the  case 
of  Melosira;  two  principal  types  were  listed,  M.  varians  and 
M.  granulata, — though  even  these  two  seem  at  times  to  intergrade. 
Other  described  species  will  be  found  among  the  individuals  thus 
distributed.  In  the  case  of  Difflugia  globulosa  and  D.  lobostoma  a 
large  number  of  intergrading  and  variable  forms  are  included.  It 
would  be  possible  to  find  among  these,  representatives  of  many 


12 

recently  described  species.  In  these  instances  the  difficulty  lies 
not  so  much  in  finding  representatives  of  these  closely  related 
species,  but,  rather,  in  drawing  the  lines  between  them  and  placing 
every  individual  enumerated  in  the  proper  pigeon-hole.  To  avoid 
this  difficulty,  the  separation  was  not  attempted  in  every  case. 
With  the  hope  that  the  results  would  throw  some  light  on  the  ques- 
tion of  seasonal  variation,  this  separation  was  attempted  in  the 
genus  Brachionus,  where  the  species  characters  are  confined  to 
prominent  structural  features. 

So  far  as  it  was  feasible,  specific  distinctions  wrere  accepted  as 
found,  and  utilized  whenever  possible.  In  the  lists  and  discussions 
which  follow,  the  inclusion  of  a  species  does  not  necessarily  carry 
with  it  the  inference  that  it  is  regarded  by  the  writer  as  valid  or 
well  founded.  It  merely  represents  in  our  enumerations  a  more  or 
less  continuous  succession  of  organisms  which  conform  approxi- 
mately to  the  descriptions  and  figures  of  the  species  designated  by 
the  name  in  question.  Inferences  regarding  the  rank  or  validity 
of  the  species  reported  will  be  given  whenever  the  statistical  data 
or  my  observations  on  the  variability  of  the  organism  seem  to 
afford  data  bearing  on  the  standing  of  the  species.  While  not  a 
few  of  the  species  reported  may  justly  be  regarded  as  synonyms,  an 
effort  has  been  made  to  use  only  names  which  represent  valid 
species  or  at  least  a  variety  or  a  seasonal  form. 

COMPARISON    OF    FRESH-WATER    AND    MARINE    PLANKTON. 

The  plankton  of  fresh  water  is  very  generally  composed  of  an 
assemblage  of  organisms,  of  plants  and  animals,  principally  crypto- 
gams and  invertebrates.  Not  all  orders  are  represented,  and  those 
that  do  occur  vary  greatly  in  the  number  of  their  representatives. 
The  fresh-water  plankton  differs  from  that  of  the  sea  in  the  almost 
universal  absence  of  larval  forms,  in  the  smaller  number  of  inverte- 
brate groups  represented,  and  in  the  smaller  size  of  its  component 
organisms.  Fresh-water  plankton  has  almost  no  limnetic  coelen- 
terates,  Hydra  fusca  being  the  only  representative  as  yet  discovered 
in  our  locality.  The  absence  of  the  larger  Crustacea,  of  limnetic 
mollusks  and  worms,  and  of  tunicates  and  Radiolaria  robs  limnetic 
life  of  the  diversity  found  among  pelagic  organisms  of  the  sea. 
The  only  larval  stages  found  in  our  locality  are  the  glochidia  of  the 


13 

Unionidae,  whose  limnetic  sojourn  is  at  the  best  but  brief,  and  the 
larvas  of  certain  dipterous  insects,  such  as  Chironomus  and  Corethra. 
The  limnetic  habit  of  these  larvas  is  hardly  established  as  yet.  The 
small  size  of  fresh-water  planktonts  as  contrasted  with  those  of  the 
sea  is  very  striking.  Representatives  of  the  same  group — for  exam- 
ple, the  Dinoflagellata  and  the  Entomostraca — in  the  two  habitats 
exhibit  this  contrast.  The  largest  entomostracan  of  fresh  water  is 
less  than  a  centimeter  in  length,  and  there  is  nothing  to  compare 
with  the  pelagic  ccelenterates,  Mollusca,  or  such  tunicates  as  Salpa 
and  Pyrosoma.  The  smaller  size  of  fresh- water  planktonts  may 
be  due  to  the  lower  specific  gravity  of  the  environing  medium,  and 
perhaps  also  to  the  effect  of  smaller  quantities  of  dissolved  salts 
upon  the  metabolic  processes  of  limnetic  animals. 

Notwithstanding  this  absence  of  large  individuals  in  the  plank- 
ton of  fresh  water,  the  total  quantitative  production  of  plankton 
per  cubic  meter  is  greater  here  than  in  the  sea.  For  example,  the 
average  production  in  the  Illinois  River  is  2.71  cm.3,  and  the  average 
amount  in  adjacent  backwaters  rises  as  high  as  22.55  cm.3  (in  Phelps 
Lake) .  These  measurements  were  made  by  the  centrifuge,  and  the 
results  of  the  "Plankton  Expedition"  of  Hensen  reduced  to  this 
basis  of  measurement  by  Kramer  ('97)  show  that  the  Atlantic 
Ocean  at  the  time  of  this  expedition  had  in  the  upper  strata  exam- 
ined but  0.12  to  0.48  cm.3  of  plankton  per  cubic  meter 

ORGANISMS    OF    THE    PLANKTON. 

The  groups  of  plants  represented  in  the  plankton  of  the  Illinois 
River  are  principally  algas,  of  which  the  Bacteriacea  are  but  partially 
retained  in  the  collections  and  are  usually  omitted  in  plankton 
investigations.  The  Sckizophyce&,  or  blue-green  algae,  furnish  a 
few  important  representatives  and  a  number  of  adventitious  .species. 
The  ChlorophycecB,  or  green  algas,  on  the  other  hand,  abound  both 
in  species  and  individuals,  and  afford  an  element  of  great  impor- 
tance in  the  primal  food  supply.  The  Bacillariacece  are  exceedingly 
abundant,  and  are  represented  by  a  number  of  eulimnetic,  as 
well  as  many  adventitious,  species.  They  also  constitute  one  of 
the  primal  sources  of  food  for  the  zooplankton.  The  Conjugate 
furnish  but  few  species  and  individuals — -principally  desmids — to 
the  phytoplankton.  The  phanerogams  afford  a  few  species  which 


14 

are  often  taken  with  the  plankton  by  virtue  of  their  semi-limnetic 
habit,  but  do  not  in  the  living  state   enter   the  food  cycle  of  the 
plankton  nor  affect  its  economy  except  as  competitors. 

The  zooplankton  includes  representatives  of  a  considerable 
range  of  groups,  though  both  in  species  and  individuals  the  Proto- 
zoa, Rotifer  a,  and  Entomostraca  predominate  among  the  animals. 
Representatives  of  other  groups  are  in  the  main  adventitious. 

Among  the  Protozoa,  the  Rhizopoda  are  constantly  represented 
by  many  individuals  and  a  considerable  number  of  species,  many 
of  which  may  be  adventitious,  but  most  of  which  are  wont  to  adopt 
the  limnetic  habit  during  the  warmer  months.  The  Heliozoa  are 
few  both  in  species  and  individuals.  The  Mastigophora  (which  in 
our  discussions  include  all  green  and  brown  flagellates  often  clas- 
sified with  the  Chlorophyce®  and  Ph&ophycece]  vie  with  the  Chloro- 
phycecs  and  Bacillariacecz  for  the  first  place  as  converters  of  the 
inorganic  (and  perhaps  also  the  dissolved  organic)  matter  into  food 
for  the  zooplankton.  They  are  exceedingly  numerous  in  our  plank- 
ton both  in  species  and  individuals,  and  form  quantitatively  a  con- 
siderable part  of  the  plankton  during  the  summer  months.  The 
usual  method  of  plankton  collection — by  silk  bolting-cloth — per- 
mits a  large  proportion  of  these  organisms  to  escape.  The  Ciliata 
furnish  a  few  constant  members  of  the  plankton,  and  numerous 
adventitious  and  parasitic  species.  During  the  low  water  of 
autumn,  when  bacterial  contamination  is  at  its  height,  these  organ- 
isms form  a  large  part  of  the  plankton.  The  small  size  of  some  of 
the  ciliates,  combined  with  their  motility  and  flexibility,  renders 
the  loss  by  their  escape  through  the  silk  net  considerable.  The 
Suctoria  furnish  but  few  species  and  individuals — mainly  adventi- 
tious or  attached  to  other  planktonts. 

The  Rotifera  constitute,  both  in  species  and  .individuals,  the 
most  important  single  group  of  analytic  organisms,  that  is  those 
of  distinctly  animal  metabolism,  occurring  in  our  plankton.  This 
may  in  part  be  due  to  our  shallow  warm  waters  and  to  the  abundance 
of  Chlorophyceoz  and  Mastigophora,  which  enter  largely  into  their 
food.  This  abundance  of  the  Rotifer  a  may  prove  to  be  character- 
istic of  the  plankton  of  rivers  (  potamoplankton)  as  contrasted 
with  that  of  lakes  (limnoplankton) .  While  many  rotifers  are 
eulimnetic,  the  plankton  also  contains  numerous  adventitious 
species. 


15 

The  Entomostraca  include  the  largest  fresh-water  planktonts, 
and  in  every  respect  constitute  an  important  element  of  our  river 
plankton.  They  form  the  final  link  in  the  food  cycle  which  con- 
nects the  nutrients  in  solution  in  the  water  and  in  decaying  detritus 
with  the  fish  and  other  aquatic  vertebrates.  They  include  numer- 
ous species,  some  of  which  are  adventitious.  All  of  the  Ostracoda 
belong  to  this  latter  class.  The  Cladocera  furnish  some  of  the 
most  important  eulimnetic  species  and  a  large  number  of  adventi- 
tious forms,  while  the  Copepoda  are  almost  wholly  eulimnetic. 

In  addition  to  these  groups,  the  Turbellaria,  Oligoch&ta,  Hexap- 
oda,  Hydrachnida,  Gastrotr-icha,  and  Bryozoa  furnish  a  few  species 
and  individuals  of  a  semi-limnetic  or  adventitious  character  to  the 
plankton. 

In  the  table  which  follows,  these  various  groups  are  listed,  and 
the  number  of  forms  occurring  in  each  is  noted.  In  order  to  give 
some  idea  of  the  proportionate  representation  of  these  groups  in 
our  plankton,  the  table  includes  the  sum  of  the  number  of  indi- 
viduals per  m.3  of  water  in  the  weekly  collections  for  the  year  1898. 
This  was  a  year  of  no  marked  departure  from  the  normal  regimen 
of  hydrographic  conditions  (Part  I.,  PI.  XII.).  The  summer  and 
autumn  flushes  tend  to  lower  the  population  somewhat  below  that 
of  more  stable  seasons,  but  beyond  this  feature  there  is  nothing  to 
suggest  that  the  plankton  of  this  year  may  not  represent  a  fair 
average  of  that  recurring  each  year  in  the  Illinois  River.  The  fig- 
ures given,  in  all  cases  refer  to  the  number  of  individuals  per  cubic 
meter  (excepting  only  such  cases  as  Synura  and  Uroglena,  where 
the  colony  rather  than  the  individual  becomes  the  unit).  The  algae 
and  Protozoa  include  many  species  enumerated  in  filter-paper  col- 
lections, which  accounts  for  the  large  numbers  in  some  of  the  totals. 
The  "number  of  forms"  listed  refers  to  the  total  number  found  in 
the  waters  of  the  river  during  the  period  of  our  operations.  Some 
species  not  noted  in  1898  are  therefore  included.  Unidentified 
forms  are  not  included  in  the  list  of  number  of  species,  though  the 
groups  here  listed  to  which  they  belong  were  known.  Some  forms 
referred  to  genera  but  not  determined  as  to  species  are,  however, 
included. 

This  table  throws  some  light  upon  the  ecological  relations  of 
the  groups  composing  the  plankton,  since  it  gives  some  clue  to  their 
relative  numbers,  and  these  condition  in  a  general  way  the  food 


16 

relations  existing  between  the  different  groups.  The  plants  are 
more  abundant  (and  generally  smaller)  than  the  animals,  outnum- 
bering them  nearly  5  to  1 .  Computation  shows  that  for  each  one  of 
the  Cladocera  there  are  7  Copepoda,  the  predominance  of  the  latter 

CONSTITUENT   GROUPS   OF    THE    ANNUAL    PLANKTON    OF    THE    ILLINOIS    RIVER. 
AVERAGE  OF  52   WEEKLY  COLLECTIONS  IN   1898 — NUMBER  PER  M3. 


Number  of 

forms 
recorded. 


Number  of 
individuals. 


Algae: 
Bacteriaceae  

3 

(57,142,822)* 

Schizophyceae  

9 

85,909,985 

Chlorophyceae  

33 

53,175,105 

Bacillariacece  

29 

396,192,716 

Conjugatas  

7 

48,459 

Phanerogamia  

2 

9 

Total  phytoplanktonts  

83 

535,326,274 

Protozoa  —  total  

(185) 

(111,731,000) 

Mastigophora  

68 

95,856,449 

Rhizopoda  

59 

55,364 

Heliozoa  

5 

4,871 

Sporozoa  

3 

1,638 

Ciliata  

45 

15,812,346 

Suctoria  

5 

332 

Rotifera  

104 

592,416 

Entomostraca  —  total  

(43) 

(47,041) 

Cladocera  

26 

6,242 

Ostracoda  

4 

191 

Copepoda  

13 

40  ,  608 

Miscellaneous  .    . 

114 

9,393 

Total  zooplanktonts  

446 

112,379,850 

Total  planktonts  enumerated. 

529 

647    706   124 

Synthetic  (chlorophyll-bearing)  

613  017,986 

Analytic  (non-chlorophyll-bearing)  

34  687   781 

being  accounted  for  in  part  by  the  fact  that  their  larval  stages  are 
free-s  Dimming  and  appear  in  the  enumerations,  while  the  young  of 
the  Clidocera  are  not  set  free  until  nearer  maturity.  About  10  to 
20  per  cent,  of  the  Copepoda  are  adults.  The  relative  numbers  of 


*  Represents  fragments  of  filaments,  and  is  not  included  in  totals. 


17 

the  two  groups  are  not  so  disproportionate  as  the  figures  might 
seem  to  indicate.  For  each  one  of  the  Cladocera  there  are  95  roti- 
fers and  almost  18,000  Protozoa.  The  latter  are  distributed  as  fol- 
lows: There  are  9  rhizopods,  almost  2,400  ciliates,  and  over 
15,000  flagellates  for  each  one  of  the  Cladocera.  There  are  also 
about  86,000  plants  for  each  of  these  Cladocera.  Of  these  plants, 
64,000  are  diatoms,  14,000  are  Schizophycecz,  9,000  Chlorophycece, 
while  but  8  are  desmids.  The  great  abundance  of  diatoms,  of 
green  and  blue-green  algae,  and  of  chloryphyll-bearing  flagellates 
affords,  it  would  seem,  an  abundant  food  supply  for  the  zooplankton. 
If  of  the  Mastigophora  the  colorless  flagellates  only  be  retained  in 
the  zooplankton,  and  the  remainder — which  are  predominantly 
synthetic  forms — be  included  with  the  phytoplankton,  we  find  the 
latter  outnumbering  the  analytic  organisms  (zooplankton)  18  to  1. 
Quantitative  values  in  the  matter  of  food  relationships  are  not 
readily  determined  except  by  a  combination  of  the  chemical  and 
experimental  method.  These  results  by  the  statistical  method 
express,  with  more  or  less  error,  the  equilibrium  of  the  biological 
components  in  terms  of  the  individual  organisms. 


DISCUSSION  OF  THE  STATISTICAL  DATA  OF  THE  SPECIES  COMPOSING 
THE  PLANKTON  OF  THE  ILLINOIS  RIVER  IN  1894-1899. 

In  the  following  pages  the  organisms  occurring  in  the  plankton 
of  the  Illinois  River  will  be  recorded,  and  from  the  statistical 
data  accumulated  by  the  enumeration  method,  facts  pertaining  to 
their  relative  abundance,  seasonal  distribution,  and  periods  of  max- 
imum occurrence  will  be  cited.  The  average  number  per  cubic 
meter  for  the  year  1898  will  be  given,  based  upon  the  averages  of 
52  collections  distributed  regularly  throughout  the  year  (Part  I., 
Table  III.).  This  year  is  chosen  because  of  the  regularity  of  the 
times  of  collection  and  the  absence  of  any  considerable  irregularity 
in  the  hydrograph.  Statements  concerning  seasonal  distribution, 
etc.,  are  based  upon  the  records  for  all  the  years — 1894-1899.  All 
figures  pertaining  to  species  or  groups  marked  with  an  asterisk, 
and  starred  figures  elsewhere,  are  based  upon  filter-paper  catches; 
all  others,  upon  those  of  the  silk  net.  Temperatures  are  in  Fahren- 
heit, and  are  of  surface  waters  at  time  of  collection.. 

The  margin  of  error  in  statistical  work  of  this  sort  is  confessedly 
large.  The  complex  character  of  the  data  with  which  I  am  deal- 
ing, and  especially  the  extreme  range  in  numbers,  have  made  it 
necessary  that  I  should  adopt  some  consistent  method  of  treating 
the  computations.  I  have  therefore  chosen  to  carry  out  the  num- 
bers to  units,  as  the  most  feasible  method  of  avoiding  confusion  in 
the  handling  of  the  data.  The  use  of  round  numbers  would  have 
been  just  as  accurate.  Computation  to  units  is  therefore  to  be 
understood  as  a  matter  of  convenience,  and  not  as  an  effort  to 
exhibit  a  false  and  unattainable  accuracy. 

CRYPTOGAMIA. 
BACTERIACE^;.* 

Records  were  kept  of  the  masses  of  the  larger  members  of  this 
group  which  occurred  in  our  plankton  catches.  They  were  princi- 
pally the  dichotomously  branched  brownish  fragments  of  Creno- 
thrix,  filaments  of  Beggiatoa,  and  colonies  of  Micro  coccus.  The 
average  number  recorded  for  this  year  was  57,142,822,  and  they 
occur  throughout  the  year  in  every  collection,  rarely  falling  below 

18 


19 

10,000,000  per  m.3,  and  reach  their  maximum  development  (over 
600,000,000)  in  winter  months  (December  to  February),  especially 
during  low  water  and  more  stable  conditions,  as  in  January,  Feb- 
ruary, and  December,  1898  (Pt.  I.,  PL  XII.).  At  such  times  the 
temperature  is  at  or  near  32°.  With  flood  conditions  and  rise  in 
temperature  the  numbers  fall  below  100,000,000,  Tunning  from 
10,000,000  to  50,000,000  during  most  of  the  summer.  The  decline 
is  due  in  part  to  the  dilution  by  flood  waters,  and  largely  to  the 
retreat  up  the  stream  of  the  crest  of  the  wave  of  bacterial  activity 
caused  by  the  Peoria  pulse  of  sewage.  As  noted  in  the  discussion 
of  the  chemical  conditions,  in  Part  I.,  this  wave  lies  considerably 
above  Havana  during  the  warmer  months.  Summer  floods,  as  in 
June  and  September,  1897,  are  wont  to  wash  into  the  river  large 
quantities  of  these  organisms,  bringing  the  numbers  up  to  300,- 
000,000  at  times.  The  figures  above  cited  give  but  a  feeble  repre- 
sentation of  the  real  conditions  in  the  river  during  this  period  of 
maximum.  Many  of  these  organisms  become  attached  to  objects 
along  shore,  and  accumulate  in  great  quantity  in  quieter  waters 
along  the  channel.  They  form  a  serious  menace  to  the  fishing 
industry,  since  they  accumulate  in  a  day  or  two  upon  the  fyke-nets 
in  quantity  so  great  that  their-  weight  and  resistance  to  the  current 
are  sufficient  to  break  down  the  nets.  Their  effect  upon  the  consti- 
tution of  the  plankton  is  seen  in  the  marked  increase  in  certain 
ciliates  which  accompanies  the  maximum  of  these  organisms. 

SCHIZOPHYCEyE. 

Nine  forms  were  recorded,  though  a  number  of  others  which 
occurred  but  rarely  in  the  plankton  remained  unidentified.  The 
average  number  (combined  silk  and  filter-paper  records,  but  omit- 
ting the  former  when  the  latter  are  available)  is  85,909,985  per  m.3 
This  group  contributes  to  the  plankton  throughout  the  year,  and 
though  numerically  abundant  is  quantitatively  less  important, 
owing  to  the  small  size  of  its  most  abundant  member,  Microcystis. 
This  species  and  Oscillatoria  constitute  quantitatively  the  greater 
part  of  the  blue-green  algas  of  the  plankton.  In  contrast  with  the 
plankton  of  Lake  Michigan,  there  is  a  noticeable  decrease  in  the 
proportion  of  AnabcBua  and  Clathrocystis.  Rivularia,  Gloiotrichia, 
and  Aphanizomenon  flos-aquce,  often  reported  in  fresh- water  plank- 

(3) 


20 

ton,  were  not  found  in  our  fluviatile  environment.  This  group 
contributes  to  the  water-bloom,  contains  a  number  of  adventitious 
planktonts,  and  is  one  of  the  primal  sources  of  the  food  supply. 
In  our  waters  it  seems  to  be  quantitatively  much  less  important 
than  either  the  Chlorophycea,  the  BacillariacecE,  or  the  synthetic 
Mastigophora. 

DISCUSSION    OF    SPECIES    OF    SCHIZOPHYCE^E. 

Anabozna  spiroides  Klebahn.* — Average  number,  637,692  (silk 
15,431).  In  the  water-bloom  from  the  last  of  June  till  the  end  of 
October.  Not  noted  in  1898,  but  not  infrequent  in  1897 — a  low- 
water  year.  Temperature  range,  60°-89°.  Data  insufficient  to 
determine  maximum.  Largest  number  recorded,  7,200,000,  June 
28. 

Clathrocystis  ozruginosa  (Kutz.)  Henfr. — Average  number  of 
colonies  or  masses,  83.  More  abundant  in  the  previous  low- water 
year.  From  May  till  the  end  of  November  in  the  water-bloom. 
Predominantly  a  midsummer  species.  Maximum  in  August  and 
September  (108,000) .  Confined  principally  to  the  low  water  of  mid- 
summer, appearing  when  the  water  reaches  a  temperature  of  70°, 
and  reaching  its  maximum  development  in  temperatures  above 
this  point,  declining  at  once  to  small  numbers  (less  than  1,000) 
when  the  temperature  falls  below  60°,  but  lingering  till  the  water 
approaches  the  freezing  point  late  in  November. 

Merismopedia  glauca  (Ehrbg.)  Nag. — Average  number  of  col- 
onies, 93.  In  1897,  889,412.*  In  the  water-bloom.  Recorded 
from  July  till  the  end  of  October,  and  also  singly  in  January  and 
February.  It  was  more  abundant  in  1897  than  in  1898,  and  the 
maximum  number  (15,840,000*)  appeared  on  August  31. 

Microcystis  ichthyoblabe  Kutz.* — Average  number,  83,059,615. 
Recorded  in  all  collections  throughout  the  year,  except  in  some 
flood  waters  of  February  and  March,  when  the  silt  probably  ob- 
scures it.  Minimum  numbers  (less  than  50,000,000)  prevail  during 
cold  months,  November  to  April,  when  the  temperature  ranges 
from  32°  to  50°.  A  well-sustained  pulse  exceeding  200,000,000 
appears  with  the  volumetric  plankton  maximum  of  April-May 
(Pt.  I.,  PI.  XII.)  and  declines  to  the  previous  minimum  with  the 
falling  off  in  the  plankton.  The  maximum  pulse  appears  later,  in 
August  and  September  in  1898,  in  September  and  October  in  1897, 


21 

averaging  about  200,000,000,  and  reaching  1,697,000,000  August  9, 
1898.  The  temperatures  during  these  pulses  are  above  60°,  and 
the  period  of  the  maximum  comes  toward  the  close  of  that  of  max- 
imum summer  temperatures,  and  sometimes  in  the  autumn  decline 
(Pt.  I.,  PL  XI.  and  XII.),  when  low  and  often  stable  jriver-levels 
usually  prevail.  A  vernal  and  an  early  autumnal  pulse  are  thus 
both  present  in  the  distribution  of  this  species.  It  is  not  improb- 
able that  other  species  than  the  one  named  have  been  included  in 
the  enumeration  along  with  it  on  account  of  the  small  size  and  lack 
of  striking  characteristics.  There  are  suggestions  of  recurrent 
pulses  at  intervals  of  2-6  weeks  in  the  records  (Table  I.). 

Oscillatoria  spp. — Average  number,  15,431  (filter-paper,  637,692). 
The  probable  inclusion  of  several  species  in  the  sums  under  this 
heading  may  account  in  part  for  the  irregularity  of  the  seasonal 
curve.  Oscillatoria  has  appeared  in  every  month  of  the  year, 
though  the  occurrences  were  most  frequent  in  the  period  from  July 
till  the  first  of  October.  The  numbers  are  exceedingly  irregular 
and  variable,  and  the  pulses  of  numbers  seem  to  attend  the  initial 
stage  of  floods  following  stable  conditions.  Thus,  while  these 
organisms  occurred  but  singly  or  sparingly  in  the  plankton  during 
the  autumn  of  1897,  they  rose  to  277,200  with  the  flood  of  January 
11,  1898,  doubtless  torn  loose  by  the  current  from  the  bottom—- 
their normal  habitat.  They  are  thus  usually  adventitious  addi- 
tions to  the  plankton.  Their  frequent  irruption  into  the  plankton 
during  midsummer  and  early  autumn,  and  to  some  extent  at  other 
times,  is  due  in  part  to  the  evolution  of  marsh  gas  in  the  detritus  on 
the  bottom.  This  breaks  up  the  mats  of  Oscillatoria  which  coat 
the  bottom  and  distributes  them  through  the  upper  levels,  where 
they  remain  in  suspension  for  some  time.  This  phenomenon  is 
more  prevalent  in  the  marshy  backwaters  than  it  is  in  the  river. 
Flood  invasion  in  midsummer  into  the  backwaters,  such  as  Quiver 
Lake,  is  wont  to  cause  there  stagnation  and  great  increase  in  Oscil- 
latoria, which  to  some  extent  enters  the  river  with  the  run-off  of  the 
flood.  Movements  in  the  water  and  the  evolution  of  marsh  gas  are 
thus  principally  responsible  for  the  presence  of  Oscillatoria  in  the 
plankton.  It  still  remains  possible  that  its  flotation  during  periods 
of  optimum  conditions  of  growth  may  be  due  to  internal  physio- 
logical conditions  which  lower  the  specific  gravity  of  the  organism. 
Its  great  abundance  at  times  in  upper  levels  in  the  backwaters  sug- 


22 

gests  the  action  of  this  factor,  and  if  this  be  true,  it  becomes  a  tem- 
porary rather  than  an  adventitious  planktont.  Temperatures  seem 
to  bear  little  relation  to  the  occurrence  of  Oscillatoria  in  the  plankton. 

Tetrapedia  emarginata  Schrod.* — Average  number,  242,308. 
From  the  first  of  August  till  the  end  of  October  in  numbers  from 
1,000,000  to  3,500,000  per  m.3,  appearing  later  and  in  larger  num- 
bers in  October  in  1897  than  in  1898.  At  temperatures  above  65°. 

Tetrapedia  gothica  Reinsch,  Glceocapsa  polydermatica  Kutz.,  and 
Gloeocapsa  sp.  were  recorded  once  or  twice  in  the  midsummer  plank- 
ton in  relatively  small  numbers. 

CHLOROPHYCEyE. 
(Plates  I.  and  II.) 

Average  number,  53,175,105,  including,  without  duplication, 
species  from  both  silk  and  filter-paper  collections.  In  1897  this 
was  very  much  greater  (139,739,850),  owing  to  the  prolonged  low 
water  and  higher  temperatures  of  the  late  autumn.  Although 
abundant,  these  organisms  are  outnumbered  by  the  diatoms  six  to 
one,  and  by  the  synthetic  Mastigophora  by  about  two  to  one.  The 
ChlorophycecB  of  the  plankton,  with  few  exceptions,  are  minute,  and 
generally  escape  through  the  silk  net.  Pediastrum  and  colonies  of 
Botryococcus  are  about  the  only  species  of  which  the  usual  method 
of  plankton  collection  in  our  waters  affords  a  fair  representation. 

The  Chlorophycea  appear  in  every  collection  examined  through- 
out all  the  years  of  our  operations,  with  the  exception  of  eight  in 
midwinter  floods  in  1895  and  1896.  As  a  group  they  are  adapted  to 
the  whole  range  of  temperatures,  and  exhibit  in  1897,  on  April  28, 
a  well-defined  vernal  pulse  of  367,200,000,  and  a  series  of  autumnal 
pulses  culminating  September  21  at  216,000,000,  October  19  at 
367,200,200,  and  November  23  at  52,000,000.  In  this  year  the 
-midsummer  pulses  are  of  minor  importance  in  comparison  with 
those  of  spring  and  autumn.  In  1898  the  vernal  pulse  is  also  well 
defined,  culminating  May  3  at  212,406,400,  and  it  is  followed  by  a 
series  of  four  midsummer  pulses  of  considerable  magnitude,  wrhich 
culminate  June  14  at  46,000,000,  July  19  at  277,000,000,  August  9 
at  370,000,000,  and  August  30  at  189,000,000.  The  autumnal  pulse 
appears  September  27,  attaining  70,526,400.  The  summer  and 
autumn  hydrographs  of  this  year  are  much  more  disturbed  than  in 


23 

the  previous  year  (cf.  PL  XI.  and  XII.,  Pt.  I.),  especially  at  the  time 
of  the  autumnal  pulse.  This  may  account  for  the  contrast  in  the 
two  years.  The  Chlorophycea  as  a  whole  exhibit  (PI.  I.  and  II.  and 
Table  I.)  the  tendency  to  form  a  seasonal  curve  of  recurrent  pulses 
at  approximately  monthly  intervals  (three  to  six  weeks)-which  gen- 
erally coincide  with  those  of  other  chlorophyll-bearing  organisms. 

Thirty-three  forms  of  Chlorophycecz  were  recorded,  and  closer 
inspection  of  the  collections  will  undoubtedly  yield  a  considerable 
additional  number  either  of  closely  related,  and  therefore  included, 
species,  or  of  those  which  occur  but  occasionally  or  in  small  numbers 
in  the  plankton. 

Numerically  the  leading  species  in  the  order  of  their  importance 
are  Scenedesmus  quadricauda,  Crucigenia  rectangularis,  Actinastrum 
hantzschii,  Raphidium  polymorphism,  Scenedesmus  genuinus,  S.  obli- 
quus,  Richteriella  botryoides,  Ophiocytium  capitatum,  Oocystis  naegelii, 
Ccelastrum  cambricum,  Oocystis  solitaria,  and  Schroederia  setigera. 
With  the  exception  of  Botryococcus  braunii  and  the  species  of  Pedias- 
trum,  the  remaining  forms  are  both  quantitatively  and  numerically 
of  minor  importance.  The  species  just  named  were  enumerated  only 
in  the  silk-net  collections,  and  ccenobia  rather  than  individual  cells 
were  listed.  If  allowance  is  made  for  the  loss  of  small  individuals 
through  the  silk,  and  for  the  increase  that  would  follow  if  individ- 
uals rather  than  ccenobia  were  the  basis  of  representation,  Pedi- 
astrum  would  occupy  a  place  in  the  front  rank  of  importance  in  the 
ChlorophycecB  of  the  plankton  numerically  as  well  as  quantitatively. 
As  quantitative  factors  in  the  ecology  of  the  plankton,  Pediastrum, 
Scenedesmus,  Ccdastrum,  and  Botryococcus  take  precedence  over  the 
smaller,  though  more  numerous,  forms,  such  as  Raphidium  and 
Crucigenia. 

The  group  is  thus  well  represented  in  our  plankton  both  in 
species  and  individuals.  The  leading  planktonts  of  the  group 
reported  in  European  and  other  waters  in  lakes  and  rivers  are  here 
represented  almost  without  exception  by  identical  or  closely  related 
species.  Botryococcus  alone  seems  to  be  less  abundant  than  in 
lakes — at  least,  according  to  my  own  observations,  it  is.  much  more 
abundant  in  the  summer  plankton  of  Lake  Michigan  than  in  that  of 
the  Illinois  River.  The  maximum  numbers  of  Pediastrum  reported 
by  Apstein  ('96)  for  Dobersdorfer  See  in  July,  when  reduced  to 
number  per  m.3,  are  frequently  equaled  or  surpassed  in  our  waters. 


24 

Data  for  comparisons  in  the  case  of  the  more  minute  organisms 
which  escape  the  silk  are  lacking,  since  results  of  supplementary 
methods  have  not,  up  to  the  present,  been  published  elsewhere. 
It  seems  probable,  however,  that  the  Chlorophycecs  will  be  found  to 
be  somewhat  more  characteristic  of  the  plankton  of  rivers  than  of 
lakes,  and  to  be  more  prevalent  wherever  the  shore  with  its  decay- 
ing vegetation  forms  a  large  factor  in  the  environment  or  where 
sewage  contamination  affords  the  requisite  food  for  their  develop- 
ment. 

DISCUSSION    OF    SPECIES    OF    CHLOROPHYCE^E. 

Actinastrum  hantzschii  Lagerh.* — Average  number,  199,038 
(silk  net,  338).  From  May  until  the  middle  of  November,  with 
maximum  of  21,600,000  on  August  30,  1898,  and  of  122,000,000  on 
September  21,  1897.  There  are  also  indications  of  a  vernal  pulse, 
which  on  May  25,  1897,  attained  90,000,000.  The  major  pulse 
occurs  late  in  the  summer,  in  August  and  September,  while  dimin- 
ished numbers  continue  until  the  first  of  November.  Three  single 
occurrences  were  noted  in  January,  1898,  following  the  unusual 
prevalence  of  1897,  but  aside  from  these  the  species  occurs  in  the 
plankton  at  temperatures  above  45°,  and  both  pulses  lie  in  temper- 
atures above  65°.  As  in  many  other  species,  a  greater  development 
was  attained  in  1897,  in  stable  low  water,  than  in  1898  in  disturbed 
hydrographic  conditions.  This  species  occurs  in  the  water-bloom,  is 
favored  by  stable  conditions,  and  finds  its  optimum  temperature 
between  65°  and  80°. 

Botryococcus  braunii  Kiitz. — Average  number  of  colonies,  75.  In 
previous  years  it  was  much  more  abundant,  averaging  3,300  in  1897. 
It  occurs  from  the  first  of  April  well  into  October,  though  in  1897  it 
continued  until  the  middle  of  December.  It  may  thus  appear 
throughout  the  whole  range  of  temperatures,  32°  to  90°,  but  as  a 
rule  occurs  above  60°.  There  is  a  suggestion  of  a  minor  pulse  in 
June,  1896,  but  not  in  other  years.  The  major  pulse  attains 
57,200  on  August  15,  1896,  and  42,000  on  September  14,  1897,  and 
appears,  with  smaller  numbers,  in  August  of  preceding  years.  The 
species  occurred  but  sparingly  in  1898.  It  is  found  in  the  wTater- 
bloom,  and  is  more  abundant  in  the  backwaters  than  in  the  main 
stream. 

Ccelastrum  cambricum  W.  Archer.* — Average  number  of  cceno- 
bia,  640,384  (silk,  477).  Occurs  from  the  latter  part  of  March  till 


25 

towards  the  end  of  November,  but  principally  from  May  through 
October.  There  are  but  slight  indications  of  a  vernal  pulse,  which 
on  May  25,  1897,  culminates  at  3,600,000.  The  major  pulse  cul- 
minates at  10,800,000  on  August  9,  1898.  In  the  low  water  and 
prolonged  high  temperatures  of  1897  the  major  pulse  continues 
through  September,  culminating  on  the  21st  at  32,000,000.  The 
average  number  in  this  year  was  about  four  times  as  great  as  in 
1898.  The  temperature  limit  is  43°,  though  occurrences  are  few 
and  numbers  small  below  65°.  The  maximum  development  appears 
within  the  period  of  maximum  heat,  and  towards  its  close.  It  is 
characteristic  of  the  plankton  of  late  summer  and  early  autumn. 

Crucigenia  rectangularis  Nag.* — Average  number  of  colonies, 
7,153,846.  Recorded  in  all  months  but  March  and  April,  but  spar- 
ingly from  November  till  May.  In  1897  pulses  appeared  in  August, 
September,  and  October,  attaining  32,400,000,  57,600,000,  and 
118,800,000,  respectively.  In  1898  there  was  but  a  single  pulse — 
in  August,  of  158,400,000.  It  was  more  abundant  in  the  former 
year.  It  is  present  continuously  in  large  numbers  from  July  to 
October,  though  in  1897  the  impetus  of  the  unusual  development 
was  manifested  by  the  continuance  of  the  species  even  into  Janu- 
ary. The  optimum  temperatures  lie  above  70°,  in  the  latter  part 
of  the  period  of  maximum  heat,  though  the  species  has  been  found 
in  the  plankton  throughout  the  whole  range  of  temperatures.  The 
abrupt  decline  in  numbers  occurs  between  65°  and  40°.  It  is  char- 
acteristic of  the  plankton  of  late  summer  and  early  autumn. 

Golenkinia  radiata  Chodat. — Average  number  of  colonies, 
519,231.  It  appears  most  abundantly  during  the  April-May  plank- 
ton pulse  (7,200,000)  and  again,  in  increased  numbers,  at  the  end  of 
August,  thus  suggesting  a  vernal  and  a  late  summer  maximum.  It 
seems  to  be  most  abundant  at  about  60°,  a  temperature  somewhat 
below  the  optimum  for  the  two  preceding  species.  Two  occur- 
rences in  December,  1896,  and  large  numbers  in  August  indicate  its 
adaptability  to  the  full  range  of  temperatures. 

Oocystis  naegelii  A.  Br.*— Average  number,  207,692.  In  1897, 
much  more  abundant  (average,  4,243,235).  Present  in  numbers 
(over  5,000,000)  from  the  end  of  May  till  the  end  of  September. 
In  1897, pulses  of  10,800,000,  46,800,000,  and  24,750,000  appear  in 
May,  July,  and  September  respectively.  Both  numbers  and  oc- 
currences are  much  less  in  1898.  The  optimum  conditions  thus  lie 


26 

above  70°,  though  isolated  occurrences  in  March  and  December  in- 
dicate its  presence  throughout  the  whole  range  of  temperatures. 
It  appears  to  be  a  summer  planktont  without  the  marked  prefer- 
ence for  the  close  of  the  period  of  maximum  heat  noted  in  some 
other  ChlorophycecE. 

Oocystis  solitaria  Wittr.* — Average  number,  121,153.  In  1897 
much  more  abundant,  averaging  2,170,588.  In  this  year  it 
occurs  in  numbers  above  1,000,000  from  the  end  of  July  till  the  end 
of  October,  reaching  a  maximum  of  36,000,000  on  September  21, 
1897.  Its  optimum  conditions  occur  during  the  latter  part  of  the 
period  of  maximum  heat,  at  temperatures  approaching  80°.  It 
disappears  at  60°,  save  for  isolated  appearances  in  December,  at 
33°-  -  a  fact  which  suggests  its  persistence  in  small  numbers 
thoughout  the  year.  It  is  characteristic  of  the  plankton  of  late 
summer, — that  is,  of  low  water,  high  temperatures,  and  stable  con- 
ditions. 

Ophiocytium  capitatum  Wolle*. — Average  number,  1,465,385. 
More  abundant  in  1897,  averaging  2,858,823.  Present  from  the 
last  of  April  until  the  beginning  of  November.  There  is  some  indi- 
cation of  a  vernal  pulse,  which  on  May  25,  1897,  attains  3,600,000, 
and  on  April  26,  1898,  10,800,000.  The  major  pulse  appears  in 
late  summer  or  early  autumn,  attaining  57,600,000  on  September  21, 
1897,  and  28,800,000  on  August  9,  1898.  The  two  pulses  are 
separated  by  an  interval  in  which  occurrences  are  less  frequent  and 
numbers  smaller.  This  planktont  thus  exhibits  the  tendency 
towards  seasonal  maxima  near  the  average  temperature.  The 
greater  development  in  1897  is  followed  by  a  prolongation  of  the 
occurrences  into  November.  The  optimum  temperature  appears 
to  be  about  60°  or  above,  the  vernal  pulse  appearing  at  that  tem- 
perature, and  the  major  one  at  71°.  No  records  occur  below  46°. 

Pediastrum  boryanum  (Turp.)  Menegh. — Average  number,  4,510. 
This  alga  was  found  in  every  month  of  the  year,  though  not  in 
every  collection  examined.  The  numbers  present  fluctuate  greatly 
and  are  usually  much  less  than  those  of  P.  pertusum,  with  which.it  is 
associated,  and  with  which  it  fluctuates,  often  with  remarkable 
coincidence.  I  have  included  under  this  head  those  individuals 
in  which  the  ccenobium  is  a  plate  with  no  intercellular  spaces  or 
only  insignificant  ones.  Individuals  are  not  lacking  which  serve 
to  connect  this  species  with  P.  pertusum,  and,  indeed,  with  others 


27 

which  have  been  described  in  this  genus.  This  genus  includes  the 
most  abundant  of  the  larger  algae  in  the  plankton  of  fresh  waters, 
and  it  affords  an  attractive  field  for  the  study  of  variation  by  statis- 
tical methods  and  for  the  determination  by  the  experimental  method 
of  the  effect  of  environmental  changes  upon  structure.  The  two 
groups  of  individuals  included  here  under  P.  boryanum  and  P. 
pertusum  give  typical  curves  of  seasonal  distribution  which  are  so 
similar  that  their  combination  in  a  single  series  would  not  greatly 
modify  the  resultant  seasonal  curve.  In  the  sum  total  of  all 
collections  P.  boryanum  (1,034,000)  includes  about  one  tenth  of  the 
number  referred  to  P.  pertusum  (10,830,117). 

A  few  scattering  individuals,  generally  less  than  1,000  per  m3., 
appear  at  irregular  intervals,  during  the  colder  months,  from  the 
first  of  December  until  the  end  of  March.  The  number  increases  as 
the  temperature  rises,  and  the  species  appears  in  all  collections  un- 
til November,  when  it  again  becomes  irregular  in  its  occurrence  in 
the  plankton.  The  fluctuations  in  numbers  during  this  period  are 
very  marked,  the  pulses  of  frequency  being  set  off  by  intervals  in 
which  the  numbers  are  small.  A  slight  pulse  of  2,120  appears  on 
November  17,  1894.  In  1895  the  vernal  pulse  attains  the  very 
unusual  number  of  572,824  in  the  unusually  low  water  of  that  year, 
and  the  autumnal  pulse  of  September  5  is  but  10,600,  and  is  followed 
by  a  secondary  one  on  November  27  of  4,081,  perhaps  as  a  result 
of  the  stable  conditions  and  the  abnormally  high  temperatures 
(above  45°)  which  then  prevailed  (Pt.  I.,  PI.  IX).  In  1896  the 
vernal  pulse  culminates  May  18  at  31,164,  while  the  autumnal  pulse 
is  scarcely  visible  and  the  numbers  throughout  the  summer  are 
small,  as  a  result,  it  may  be,  of  the  repeated  floods  of  that  year 
(Pt.  L,  PI.  X).  In  1897,  with  few  vernal  data,  the  vernal  pulse  does 
not  appear,  though  a  rise  to  8,000  occurs  on  July  21.  The  major 
autumnal  pulse  culminates  on  September  14  at  14,400,  and  another 
one  on  October  12  at  6,000,  attending  the  late  autumn  of  that 
year.  In  1898  there  are  vernal  pulses — on  May  10  of  6,400  and  on 
June  14  of  32,000.  The  autumnal  pulse  on  September  27  reaches 
the  considerable  number  of  65,600.  In  the  winter  of  1898-99 
Pediastrum  was  seemingly  absent  from  the  plankton.  The  pulses 
are  thus  somewhat  irregular,  though  there  is  in  this  species  a 
suggestion  of  vernal  and  autumnal  pulses  at  corresponding 


28 

temperatures.  The  optimum  conditions  seem  to  lie  above  60° 
and  the  maximum  numbers  to  occur  at  or  near  70°. 

Pediastrum  pertusum  Kiitz. — Average  number  of  coenobia, 
44,372.  This  species  appears  in  the  plankton  in  all  months  of  the 
year  and  in  almost  all  of  our  collections.  It  is  the  most  abund- 
ant representative  of  the  Cklorophyceaz  which  is  retained  by  the 
silk  of  the  plankton  net,  and  is  quantitatively  an  important 
factor  in  the  ecology  of  the  plankton.  The  numbers  during  the 
colder  months,  from  November  to  April,  when  the  water  is  from 
32°  to  40°,  are  few,  and  the  sequence  of  their  appearance  is  fre- 
quently interrupted.  As  the  temperature  rises  in  April  the  num- 
bers increase,  and  the  vernal  pulse  culminates  in  a  maximum  in  May 
or  June.  There  is  no  indication  of  the  vernal  pulse  in  the  scattered 
collections  of  1894.  In  1895  the  pulse  is  extreme,  reaching  5,264,860 
on  June  19,  in  a  period  of  exceptionally  low  water.  In  1896  a  pre- 
liminary vernal  pulse  culminates  May  8  at  23,580  and  is  followed  on 
June  17  by  one  of  107,200.  In  1897  the  few  spring  collections  do 
not  reveal  any  vernal  pulse,  while  in  1898  a  minor  one  on  May  17 
reaches  5,600,  declines  to  600  at  the  end  of  the  month,  and  rises 
again  to  56,000  by  June  21.  These  vernal  maxima  all  occur — or  at 
least  pass  through  their  period  of  development — before  the  water 
reaches  its  midsummer  temperature  of  approximately  80°.  They 
develop  during  the  transition  from  60°to  80°  (Pt.  I.,  PI.  IX.  to  XL). 
Autumnal  pulses  during  the  decline  from  80°  to  60°  appear  on  Sep- 
tember 5,  1895,  (105,996),  on  September  30,  1896  (9,200),  on  Octo- 
ber 12,  1897  (231,200),  and  on  September  27,  1898  (259,200).  In 
addition  to  these  pulses  there  are  others  at  irregular  intervals  during 
the  summer:  on  July  30,  1894  (154,548),  on  July  2,  1896  (68,400), 
on  August  15,  1896  (22,000),  on  July  14  (289,600)  and  on  August 
31,  1897  (442,000),  and  on  August  2  (295,200)  and  30  (326,400), 
1898. 

The  optimum  conditions  of  development  thus  lie  above  60°, 
and  pulses  are  more  frequent  in  spring  and  late  summer  or  early 
autumn  near  70°,  though  they  appear  somewhat  less  frequently 
during  the  summer  in  our  maximum  temperatures  near  80°.  The 
cause  of  these  pulses  is  not  conclusively  demonstrable  from  the  data 
at  hand,  owing  in  part  to  the  interval  between  examinations. 
Daily  examinations  of  the  plankton  and  chemical  analyses  seem  to 
be  desirable  for  such  demonstration.  There  are  indications,  how- 


29 

ever,  that  certain  conditions  in  the  environment  increase  the 
amplitude  of  the  pulses  by  hastening  the  rapidity  of  reproduction  of 
these  organisms.  Of  the  fifteen  well-defined  pulses  appearing  in 
our  records  of  six  years,  all  but  three  minor  ones  occur  in  stable  con- 
ditions, such  as  pertain  to  sustained  low  water.  The  ""greater  part 
of  these  pulses,  however,  occur  in  declining  floods,  when  contribu- 
tions from  backwaters  are  considerable.  It  may  seem  ill-advised  to 
refer  to  the  conditions  of  falling  river-levels  as  "stable";  neverthe- 
less, they  are  relatively  much  more  stable  than  those  which  attend 
the  in-rush  of  silt-laden  flood- waters,  and  involve  fewer  changes  in 
factors  of  the  environment.  Save  in  the  matter  of  the  relative  con- 
tributions of  backwaters  and  of  sewage  dilution  they  resemble 
those  of  sustained  low  water.  These  Pediastrum  pulses  are  also  re- 
lated to  the  nitrate  pulses  (Pt.  I.,  PI.  XLIII.-XLV.  and  Table  X.), 
but  the  relation  is  not  uniform.  In  the  majority  of  instances  the 
pulses  of  1896-1898  (during  which  time  chemical  anaylses  are 
available)  coincide  approximately  with  the  crest  or  decline  of  in- 
crease in  nitrates.  For  example,  the  pulse  noted  on  July  17,  1896, 
of  107,200  from  a  previous  level  of  1,210  on  June  1,  follows  a  wave 
of  nitrates  'progressing  for  three  weeks  and  culminating  on  June  9  at 
3.25  parts  per  million — arise  from  1.5  (Pt.  I.,  PI.  XLIIL).  On  June 
16  the  nitrates  have  fallen  again  to  2.2,  and  on  the  23d  to  2.0,  but 
rise  on  the  30th  to  2.8.  Pediastrum  responds  to  these  changes  by 
dropping  from  107,200  on  the  17th  to  15,000  on  the  27th,  and  by 
rising  again  on  July  2  to  68,400.  Not  all  of  the  fluctuations  in  the 
two  are  concomitant.  Some  of  the  most  marked  pulses  of  Pedi- 
astrum appear  at  the  lowest  levels  of  the  nitrates.  For  example, 
that  of  August  30,  1898,  of  326,400,  follows  no  nitrate  wave,  though 
it  coincides  with  a  reduction  in  nitrates  to  the  minimum  of  .05.  On 
the  other  hand,  the  nitrites  had  just  passed  on  August  23,  an  un- 
usual pulse,  to  .42,  falling  again  on  August  30  to  .22  and  on  Septem- 
ber 6  to  .05  with  the  passing  of  the  Pediastrum  pulse.  Pulses  of 
Pediastrum  are  thus  apparently  not  dependent  for  their  develop- 
ment upon  an  abundance  of  nitrates  above  the  levels  shown  in  the 
analyses,  though  a  decline  in  these  sources  of  food  or  in  other  forms 
of  nitrogen  usually  attends  these  pulses.  Pediastrum  is  but  one  of 
many  factors  among  the  planktonts,  and  in  the  environment, 
biological  and  chemical,  concerned  in  these  changes,  and  con- 
clusive demonstration  of  its  ecological  relations  must  be  obtained 


30 

by  the  experimental  method.     The  data  here  cited  are  suggestive 
only;  not  conclusive. 

The  relation  of  Pediastrum  to  the  volumetric  pulses  of  the  plank- 
ton is  not  a  constant  one,  though  there  is  some  correspondence  in 
their  fluctuations.  The  extreme  maximum  (3,264,800)  of  June  19, 

1895,  is  coincident  with  a  plankton  pulse  of  30.42  cm.3,  but  the  num- 
ber of  collections  is  insufficient  to  show  the  relative  fluctuations  of 
the  plankton  and  Pediastrum  at  that  season.     In  May  and  June, 

1897,  and  in  October,  1898,  the  Pediastrum  pulses  culminate  shortly 
after  the  volumetric  pulses.     In  July  and  September,   1897,  arid 
in  August,  1898,  they  coincide. 

Polyedrium  trigonum  Nag.* — Average  number,  432,692.  Ap- 
pears from  June  through  September,  disappearing  when  falling 
temperatures  reach  60°.  In  1897  it  continues  through  October 
with  the  higher  temperatures  (averaging  65°)  of  that  year.  There 
are  slight  indications  of  a  September  pulse. 

Polyedrium  trigonum  forma  minus  Reinsch  and  var.  tetragonum 
(Nag.)  Rabh.,  P.  bifurcatum  Wille,  and  P.  gracile  Reinsch,  were 
also  recorded  in  a  few  collections  during  the  period  of  occurrence 
of  P.  trigonum.  They  are  all  evidently  summer  planktonts. 

Raphidium  polymorphum  Fresen.* — Average  number,  2 1 ,450,000. 
Occurs  in  every  month  of  the  year  and  in  a  majority  of  the  collec- 
tions. In  1897  a  vernal  maximum  of  201,600,000  occurs  on  April 
27  and  an  autumnal  one  of  28,800,000  on  September  21.  In  1898 
a  vernal  pulse  culminates  May  3  at  24,000,000,  and  thereafter 
throughout  the  summer  at  intervals  of  three  to  six  weeks  there 
occur  five  other  pulses,  the  greatest  of  which  culminates  July  19  at 
75 ,600,000.  A  pulse  of  90,000,000  on  a  declining  flood  in  February, 
1899,  indicates  an  adaptation  on  the  part  of  this  organism  to  the 
whole  range  of  temperatures.  A  pulse  of  25,200,000  December  3, 

1896,  further  illustrates  this  adaptability.     Records  in  1897  and 

1898,  however,  suggest  that  the  optimum  lies  above  60°.     It  is  thus 
a  perennial  planktont. 

Raphidium  longissimum  B.  Schroder. — Appeared  sparingly  in 
February,  August,  October,  and  December,  suggesting  that  it  has 
also  a  perennial  distribution. 

Richteriella  botryoides  (Schmidle)  Lemm.* — Average  num- 
ber, 6,399,705  (in  1897).  From  May  to  November,  with  a  vernal 
pulse  of  25,200,000  on  May  25,  and  an  autumnal  one  of  100,800,000 


31 

on  September  21.  Optimum  temperature  about  70°,  and  disappear- 
ing from  our  records  below  60°. 

Scenedesmus  bijugatus  (Turp.)  Kiitz.* — Average  number,  155,769. 
Sparingly  from  May  till  the  close  of  September,  with  slight  traces  of 
vernal  and  autumnal  pulses.  • 

Scenedesmus  denticulatus  Lagerh.* — Average  number,  86,538. 
A  few  occurrences  in  late  summer  and  early  autumn. 

Scenedesmus  genuimts  Kirchner.* — Average  number,  778,846. 
From  May  till  the  first  of  October,  but  continued  through  this 
month  in  1897.  Vernal  pulse  not  observed,  though  the  autumnal 
pulse  attains  28,800,000  on  September  21  and  October  26,  1897. 
Midsummer  pulses  appear  in  1897  on  July  14  (16,200,000),  August 
17  (14,400,000),  and  in  1898  on  August  9  (19,800,000).  Optimum 
temperatures  lie  above  60°,  though  an  occurrence  in  December  in- 
dicates the  adaptability  of  this  organism  to  lower  temperatures. 

Scenedesmus  obliquus  (Turp.)  Kiitz.* — Average  number,  1 ,505 ,769 
(silk,  673).  This  form  appears  in  our  records  from  the  last  of  April 
until  the  middle  of  November.  Traces  of  vernal  and  autumnal 
pulses  appear  in  both  1897  and  1898,  with  intervening  midsummer 
fluctuations  of  even  greater  magnitude.  In  1897  the  vernal  pulse 
on  May  25  reaches  3,600,000;  a  midsummer  one  on  August  10, 
5,400,000;  and  the  autumnal  one  appears  twice,  once  on  September 
21  at  28,800,000,  and  again  on  October  19  at  25,200,000.  In  1898 
the  vernal  pulse  appears  May  10  at  1,800,000;  midsummer  ones,  on 
July  19  at  10,800,000,  and  August  9  at  36,000,000 ;  and  the  autumnal 
on  September  9  at  8,100,000.  As  in  some  other  organisms,  these 
pulses  are  separated  by  intervals  of  three  to  six  weeks.  The 
optimum  temperatures  lie  above  60°,  though  development  begins 
before  that  temperature  is  reached,  and  the  impetus  of  the  autumnal 
pulse,  or  acclimatization  to  lower  temperatures,  carries  the  species 
beyond  this  limit  into  temperatures  of  45°.  There  is  a  marked 
absence  of  pulses  below  60°.  This  seems  to  be  a  summer  planktont 
with  no  marked  preference  for  the  lower  temperatures  of  spring  and 
autumn. 

Scenedesmus  quadricauda  (Turp.)  Breb.* — Average  number, 
9,276,923  (silk,  8,611).  In  this  species,  as  in  the  case  of  others  of 
the  genus  and  of  the  Chlorophycecs  generally,  the  numbers  present  in 
1897  were  much  greater  than  in  1898  (32,492,647,*  silk,  5,818). 
Prolonged  low  water  and  concentration  of  sewage  afforded  stable 


32 

conditions  and  food  requisite  for  such  development.  This  species 
appears  in  our  collections  in  every  month  of  the  year,  though  in 
much  smaller  numbers  and  less  frequently  from  November  to 
April — that  is,  below  50°.  Pulses  of  noticeable  magnitude  appear 
only  above  this  temperature,  and  usually  above  60°. 

Slight  traces  of  vernal  and  autumnal  pulses  appear  in  the  col- 
lections of  the  silk  net  in  1894-1896.  In  the  filter-paper  collections 
of  1897-1898  they  are  well  defined.  The  vernal  pulse  appears  in 
1897  on  May  25  at  46,800,000,  and  in  1898  on  May  10  at  70,200,000. 
The  autumnal  maximum  in  1897  is  remarkable  both  for  its  large 
numbers  and  its  prolongation,  culminating  twice— first  on  Septem- 
ber 21  at  151,200,000,  and  again  on  October  19  at  154,800,000. 
This  remarkable  development,  combined  with  the  stable  conditions 
and  higher  temperatures  (Pt.  I.,  PI.  XI.)  of  that  low- water  autumn, 
is  responsible  for  the  continuance  of  the  species  in  our  collections 
throughout  the  winter.  In  1898  the  species  declined  earlier,  in 
November,  and  was  but  sparingly  represented  in  collections  of  the 
winter  of  1898-1899.  As  in  other  species  of  the  genus  and  other 
Chlorophycece,  midsummer  pulses  appear  at  intervals,  often  of  four 
weeks,  but  ranging  from  three  to  six.  In  1897  these  occurred  on 
July  14  at  55,800,000  and  on  August  31  at  21,600,000.  In  1898 
they  appear  on  June  28  at  10,800,000,  on  July  19  at  79,200,000,  on 
August  9  at  39,600,000,  and  on  August  30  at  54,000,000.  At  inter- 
vals between  the  pulses  the  numbers  decrease,  and  in  the  regular 
collections  of  1898  the  minima  between  the  pulses  do  not  in  any  case 
exceed  30  percent,  of  the  adjacent  maxima,  and  are  usually  very 
much  less.  The  distribution  of  the  pulses  of  this  species  coincides 
very  closely  with  that  of  the  other  species  of  the  genus,  and  also  with 
that  of  other  Chlorophycea.  For  example,  Pediastrum  pertusum, 
the  most  abundant  of  the  larger  algas,  has  seven  of  its  thirteen  pulses 
on  the  same  dates  with  those  of  Scenedesmus  quadricauda  and  three 
others  on  adjacent  dates,  leaving  but  three  which  are  not  practically 
coincident.  The  operation  of  some  common  and  general  factor  in 
the  environment  is  suggested  by  such  phenomena. 

The  wide  seasonal  range  of  this  organism  gives  it  a'  claim  to  rank 
as  a  perennial  planktont,  though  its  quantitative  distribution 
shows  clearly  that  the  optimum  temperatures  for  its  growth  lie 
above  60°.  The  largest  number  recorded  in  1897  appears  October 
19  at  a  temperature  of  65°,  and  in  1898  on  July  19  at  84°.  It  is 


33 

thus  predominant  only  during  the  warmer  part  of  the  year;  and 
while  autumnal  and  vernal  pulses  occur,  there  is  no  sustained  mid- 
summer minimum  intervening  between  them.  The  pulses  in 
Scenedesmus  as  a  rule  follow  the  volumetric  pulses  as  shown  in  silk- 
net  catches  (Pt.  I.,  PI.  XI.  and  XII.).  Thus  in  1897,  on  September 
14  and  21,  the  plankton  measures  19.8  and  3.0  cm.3  per  m.3,  respec- 
tively, Scenedesmus  quadricauda  numbering  20,700,000  and  151,- 
200,000;  and,  again,  on  October  5  and  19  the  plankton  measures 
12.92  and  1.86  cm.3,  and  this  alga  numbers  93,600,000  and  154,- 
800,000.  Its  share  in  the  volumetric  pulses  is  thus  indirect  to  a 
large  degree,  and  is  perhaps  modified  by  food  relations. 

Schroederia  setigera  (Schroder)  Lemm.* — Average  number, 
21,450,000.  In  1897,  69,040,912.  It  appears  in  all  months  of  the 
year  and  in  almost  every  collection.  It  has  well-defined  vernal  and 
autumnal  pulses  separated  by  the  summer  period,  in  which  only 
minor  pulses  occur.  In  1898  midwinter  numbers  are  as  high  as 
those  of  midsummer.  Schroederia  is  thus  truly  a  perennial  plank- 
tont.  The  vernal  pulse  appears  in  1897  on  April  27  at  302,400,000, 
and  in  1898  on  May  3  at  150,000,000.  The  autumnal  pulse  in  1897 
culminates  on  September  21  at  565,200,000,  and  is  followed  by  sec- 
ondary culminations  on  October  26  at  136,800,000,  and  on  Novem- 
ber 23  at  203,400,000.  In  1898,  when  hydrographic  conditions 
were  less  stable,  the  autumnal  pulse  reached  only  50,400,000, — on 
September  6.  This  is  followed  by  minor  pulses,  declining  to  a  mini- 
mum in  the  following  February.  It  disappeared  in  the  collections 
with  the  flood  waters  of  March,  1899.  The  sequence  of  these  sec- 
ondary pulses  follows  much  the  same  course  as  has  been  described 
for  other  species,  namely,  maxima  at  intervals  of  approximately 
a  month  (two  to  six  weeks)  separated  by  more  or  less  sharply 
defined  minima.  There  are  twelve  such  pulses  (including  the  major 
ones)  in  1898  and  an  interval  of  seven  weeks  in  March- April  in 
which  none  occurs.  Six  pulses  appear  in  the  last  five  months  of 
1897. 

The  optimum  temperatures  as  indicated  by  the  position  of  the 
vernal  (60°  in  both  1897  and  1898,  as  shown  in  Table  III.,  Pt.  I.) 
and  autumnal  (71°  in  1897  and  79°  in  1898)  pulses  lie  between  60° 
and  80°.  This  appearance  of  the  vernal  pulse  at  a  lower  temperature 
than  the  autumnal  (usually  about  10°  lower)  is  not  confined  to  this 
species  but  is  a  general  phenomenon  among  other  Chlorophycea. 


34 

It  is  apparently  a  phenomenon  of  seasonal  acclimatization,  by  virtue 
of  which  the  low  temperatures  of  the  winter  lower  the  optimum  for 
the  vernal  pulse,  and  the  high  temperatures  of  the  summer  raise 
it  for  the  autumnal  pulse. 

Selenastrum  bibraianum  Reinsch.* — Average  number  519,235. 
Recorded  only  from  the  beginning  of  August  till  the  end  of  Novem- 
ber, and  never  in  great  abundance.  Slight  evidence  of  a  September 
pulse. 

Some  other  Chlorophycecs  have  been  included  in  the  totals  as 
"unidentified,"  and  isolated  occurrences  of  the  following  have  been 
noted:  Cerasterias  longispina  (Perty)  Reinsch,  C.  raphidioides 
Reinsch,  Dactylococcus  infusionum  Nag.,  Glceocystis  gigas  (Kiitz.) 
Lagerh.,  Staurogenia  lauterborni  Schmidle,  and  a  few  of  the  Con- 
fervacecz — which  are  probably  adventitious.  These  are  a  species  of 
Conferva,  of  Prasiola,  and  of  Ulothrix — all  of  which  appear  sparingly 
in  spring  and  autumn  planktons,  the  first-named  and  the  last 
as  minute  filaments  in  the  filter-paper  collections.  A  thorough 
analysis  of  the  unidentified  forms  would  greatly  extend  the  list  of 
species  and  varieties. 

BACILLARIACE^E. 
(Plates  I.  and  II.) 

Average  number,  396,192,716,  including,  without  duplication, 
diatoms  from  both  silk  and  filter-paper  collections.  They  were 
almost  twice  as  abundant  in  the  more  stable  conditions  in  which  the 
collections  of  1897  were  made.  The  Bacillariacece  are  more  abun- 
dant than  any  other  synthetic  group  of  organisms  in  our  plankton. 
They  exceed  (in  1898)  the  Schizophycecz  five  to  one,  the  Chloro- 
phycecs  seven  to  one,  the  desmids  eight  thousand  to  one,  and  the 
synthetic  Mastigophora  by  more  than  four  to  one.  Their  numerical 
preponderance  is,  with  the  exception  of  the  synthetic  Mastigophora, 
equaled  or  exceeded  by  their  relative  quantitative  significance  in  the 
ecology  of  the  plankton. 

They  appear  without  exception  in  every  collection,  and  their 
seasonal  distribution  in  its  main  features  is  repeated  from  year  to 
year.  There  is  a  principal  vernal  pulse  in  April-May  and  a  hiemal 
pulse  in  November-December.  Minimum  periods  separate  these 
pulses  and  are  varied  by  other  pulses,  usually  of  minor  importance, 
at  intervals,  in  1898,  of  three  to  five  weeks.  The  winter  minimum 


35 

is  at  a  lower  level  than  the  summer  one.  In.  1894  the  interval  of 
collection  is  too  great  to  follow  the  seasonal  distribution,  but  there 
are  hints  of  summer  and  autumnal  pulses.  In  1896  there  were  no 
May  collections,  and  the  largest  number,  6,060,665,  appears  June 
19,  five  minor  pulses  — on  July  18,  August  21,  September  F2,  October 
11,  and  November  5 — intervening  before  the  hiemal  pulse  of  3,574,- 
028  appears  on  November  27.  Other  pulses  follow  on  December  18, 
January  6,  February  4,  March  4,  and  March  17,  before  the  vernal 
pulse  of  1896  culminates  at  105,440,858  on  April  24.  This  is  fol- 
lowed by  minor  pulses  on  May  18,  June  ]  1,  July  18,  August  8,  and 
September  16,  and  by  the  hiemal  pulse  of  December  3  of  346,982,- 
928*.  The  vernal  pulse  of  1897  appears  April  27  at  6,207,473,520, 
but  is  surpassed  by  a  pulse  on  July  14 — principally  of  Melosira 
spinosa — of  11,459,289,600,  and  minor  pulses  then  follow  on  August 
17,  September  29,  October  26,  and  December  7  and  21.  The  hie- 
mal pulse  of  this  year  is  insignificant.  In  1898  three  minor  pulses 
appear,  January  21,  February  15,  and  March  22,  and  the  vernal 
pulse  culminates  May  10  at  3,865,257,360.  Minor  pulses  follow  on 
June  14,  July  19,  August  9,  August  30,  September  27,  October  25, 
and  November  22,  and  the  hiemal  pulse  culminates  December  15  at 
436,535,790,  followed  in  1899  by  minor  ones  on  January  10,  Febru- 
ary 14,  and  March  14. 

Some  of  the  pulses  here  indicated  are  due  to  the  development  of 
single  species,  as  that  of  Melosira  on  July  14,  1897.  Most  of  them, 
however,  are  composite,  including  a  number  of  species.  This  is 
especially  true  of  the  vernal  pulse,  wThich  in  1898  is  due  to  the  com- 
bined increase  in  Fragilaria  virescens  and  F.  crotonensis,  Cydotella, 
Asterionella,  Navicula  spp.,  and  Synedra  acus.  Asterionella  culmi- 
nates early  in  the  vernal  pulse  and  the  majority  of  the  others 
towards  its  close.  Melosira  varians  is  among  these,  but  M. 
spinosa  contributes  less'  to  this  pulse  than  it  does  to  later  ones. 
Minor  pulses  are  also  composite,  as,  for  example,  that  of  August  9, 
1898,  which  is  due  to  Melosira  spinosa,  Cydotella,  and  Navicula. 

FACTORS    CONTROLLING    DIATOM    PRODUCTION. 

The  fact  that  many  of  these  pulses  represent  the  combined 
fluctuations  of  a  number  of  species  leads  us  to  look  for  some  factor 

*  Filter-paper  collections  included  in  this  and  in  following  years. 
(4) 


36 

in  the  environment  common  to  them  all  to  which  these  pulses  may  be 
attributed.  On  the  following  page  the  seasonal  distribution  of  the 
total  diatoms  has  been  plotted  for  1898,  along  with  that  of  the  ni- 
trates and  of  the  total  plankton  (volumetric),  the  thermograph,  and 
the  hydrograph.  An  examination  of  the  changes  in  nitrates  yields 
no  marked  evidences  of  correlation.  The  vernal  pulse  of  diatoms 
follows  the  high  nitrates  of  winter  and  spring,  and  the  hiemal  pulse 
in  December  appears  after  their  autumnal  rise,  and  in  this  particular 
year  develops  at  the-  time  of  an  unusual  drop  in  nitrates  (Pt.  I., 
PI.  XLV.).  The  diatom  pulses  do  not  show  any  constant  relation 
to  the  movement  in  nitrates  either  in  amount  or  direction.  Whipple 
('94)  has  noted  the  importance  of  nitrates  in  the  development  of 
diatoms  in  reservoir  waters.  The  fact  that  little  correlation 
appears  in  our  waters  between  the  fluctuations  of  the  nitrates  and 
the  growth  of  diatoms  may  be  due  to  the  presence  here  of  nitrates 
— owing  to  sewage  contamination — far  in  excess  of  the  demands 
which  the  diatoms  make,  and  the  limitations  placed  by  other  elements 
in  the  environment  are  reached  before  that  of  the  nitrate  food- 
supply  becomes  operative.  The  distribution  of  these  diatom  pulses 
throughout  the  whole  year,  even  in  seasonal  extremes,  seems  to  pre- 
clude the  factor  of  temperature  as  the  immediate  cause  of  the 
pulses  except  as  it  may  affect  the  growth  of  individual  species, 
which  is  sometimes  apparently  the  case,  as  is  shown  in  subsequent 
pages. 

The  vernal  pulse  is  attained  each  year  about  May  1,  at  which 
time  the  water  passes  the  temperature  of  60°.  The  average  of  the 
recorded  surface  temperatures  of  1898  in  the  river  is  about  58°. 
Surface  temperatures,  except  in  winter  months,  are  usually  several 
degrees  higher  than  bottom  temperatures  (Pt.  I.,  Table  III.).  Our 
records  are  always  of  diurnal  temperatures.  The  true  average  tem- 
perature, owing  to  colder  water  at  lower  levels  and  to  the  nocturnal 
decline,  will  lie  several  degrees  below  58° — probably  about  55°. 
The  greatest  development  of  diatoms  thus  takes  place  at  a  temper- 
ature a  few  degrees  higher  than  the  average  temperature  for  the 
year.  Owing  to  the  somewhat  greater  abundance  of  diatoms  dur- 
ing the  warmer  months,  the  average  thermal  exposure  of  the  plank- 
ton diatoms  will  be  somewhat  higher  than  the  average  temperature 
of  the  year.  There  may  be  some  significance  in  this  phenomenon 
of  the  occurrence  of  the  optimum  temperature  for  development  at 


37 


Fig.  A. — Diagram  showing  the  seasonal  distribution  of  diatoms,  total  plankton, 
nitrates,  and  thermograph  and  hydrograph  of  Illinois  River  at  Havana  for  1898. 


38 

approximately  that  of  the  average  thermal  exposure.  The  vernal 
pulse  may,  in  part  at  least,  be  the  result  of  a  process  of  natural 
acclimatization.  The  fact  that  a  similar  development  does  not 
recur  when  this  temperature  is  repassed  in  the  autumnal  decline 
militates,  it  is  true,  against  the  potency  of  this  temperature  as  a 
factor  in  the  vernal  pulse.  This  temperature  is  passed  in  October 
(Pt.  I.,  PL  VIII.— XIII. ),  but  October  pulses  are  rarely  so  pronounced 
as  those  of  adjacent  months.  Other  factors  more  potent  than  tem- 
perature are  operative  at  that  season  of  the  year. 

As  will  be  seen  in  the  diagram,  the  most  pronounced  and  pro- 
longed minimum  appears  in  January,  February,  and  March.  In 
these  months  but  a  single  record  in  excess  of  100,000,000  per  m.3 
is  found.  This — or  at  least  the  first  two  months  of  it — is  the  period 
of  the  ice  blockade  (Pt.  I.,  PI.  IX.-XIIL),  during  which  the  aeration 
of  the  water  by  the  wind  is  prevented,  and  the  customary  equilib- 
rium in  gaseous  contents  may  be  disturbed.  It  is  the  time  when 
stagnation  most  threatens  disaster  to  the  plankton.  The  earlier 
stages  of  this  blockade  in  December  do  not  seem  to  be  deleterious 
to  the  growth  of  diatoms,  since  at  such  times  the  blockade  is  less 
complete,  the  exclusion  of  light  by  the  ice  less  effective,  and  the 
accumulation  of  the  products  of  decay  less  pronounced.  The  data 
at  hand  do  not  suffice  to  elucidate  the  matter  further. 

The  position  of  the  diatom  pulses  with  respect  to  the  movement 
of  the  hydrograph  is  suggestive — though  not  conclusive — of  a  pos- 
sible correlation  between  the  two  phenomena.  The  double  vernal 
pulse  of  April-May  appears  in  the  declining  waters  of  the  major 
spring  flood.  The  diatom  pulse  of  June  14  is  found  in  the  decline 
of  the  May- June  flood.  The  pulse  of  August  9  is  caught  on  the  ris- 
ing waters  of  a  slight  flush  of  the  river,  and  that  of  August  30  on  its 
decline.  That  of  September  27  appears  after  a  series  of  slight  rises, 
and  those  of  both  October  and  November  attend  rising  water,  but 
the  well-developed  pulse  of  December  appears  with  its  decline. 

There  are,  counting  the  double  vernal  pulse,  ten  pulses  in  1898, 
from  March  to  January.  Of  these,  seven  are  found  on  declining 
floods,  and  .but  three  on  rising  water,  and  two  of  these  three  appear 
during  the  slow  rise  of  October-November.  Furthermore,  the 
magnitude  of  the  flood  is  correlated  with  that  of  the  diatom  pulse. 
The  vernal  pulses  of  3,453,778,080  and  3,865,257,360  attend  the 
major  spring  flood,  culminating  April  2  at  18  feet;  the  pulse  next  in 


39 

size,  that  on  June  14  of  1,039,619,680,  attends  the  decline  of  the 
flood  next  in  importance — that  culminating  May  25  at  13.9  feet; 
while  the  third  pulse,  that  on  December  15  of  436,535,790,  attends 
the  decline  of  the  flood  culminating  November  25  at  8.7  feet.  The 
hydrograph  of  1897  (Pt.  I.,  PI.  XI.)  is  unlike  tha^of  1S98  (Pt.  I., 
PI.  XII.)  in  the  delay  of  the  so-called  "June"  rise,  which  culminates 
July  5  at  7.5  feet.  Its  decline  runs  through  the  month  into  August. 
The  diatom  pulse  attending  the  "June"  rise  of  1897  appears  about 
a  month  later  than  it  did  with  the  earlier  pulse  of  1898,  culminating 
July  14  at  1 1,459,289,600.  A  delay  in  the  flood  is  thus  attended  by' 
a  delay  in  the  diatom  pulse.  In  1897  there  is  no  December  rise  and 
no  diatom  pulse  of  noticeable  magnitude,  though  in  1895,  in  similar 
absence  of  the  flood,  there  is  a  well-defined  diatom  pulse.  In  1896 
there  is  a  series  of  five  floods,  each  involving  the  early  stages  of 
overflow  (Pt.  I.,  PI.  X.),  and  on  the  decline  of  each  occur  one  or 
more  diatom  pulses. 

It  is  but  natural  that  the  greater  number  of  diatom  pulses  should 
fall  on  declining  river-levels,  since,  as  I  have  previously  shown, 
these  periods  exceed  in  duration  those  of  rising  floods.  They  also 
predominate  during  the  prevalence  of  seemingly  favorable  temper- 
atures, and  are  characterized  by  relatively  more  stable  conditions 
in  the  environment.  There  is,  however,  it  seems  to  me,  another 
and  more  potent  reason  why  diatom  pulses  appear  at  such  times. 
It  lies  in  the  overflow  of  seed-beds  in  the  margins  of  the  permanent 
backwaters  and  the  run-ofl  of  the  plankton  which  develops  there 
with  the  fall  in  levels.  This  is  very  apparent  to  one  familiar  with 
the  locality.  During  the  decline  of  the  flood  the  channel  current  is 
often  diverted  in  minor  lateral  channels,  such,  for  example,  as  that 
(Pt.  I.,  PI.  II.)  which  courses  through  Thompson's  Lake  Slough  into 
Thompson's  Lake  and  out  again  into  the  river  at  its  southern  end  by 
way  of  "the  swale"  and  the  "cut  road."  A  similar  current  on  the 
eastern  bottoms,  which  enters  partially  by  way  of  Mud  Lake  Slough, 
rejoins  the  river  through  Quiver  Lake.  These  lateral  currents  are 
joined  by  the  run-off  from  overflowed  bottoms  and  adjacent 
marshes  and  swamps,  all  of  which,  as  well  as  the  permanent  back- 
waters thus  draining  into  the  channel,  breed  at  such  times  an  abun- 
dant plankton  including  diatoms.  The  contributory  function  of  the 
backwaters  to  the  plankton  of  the  river  proper  is  thus  at  its  maxi- 
mum during  the  decline  of  the  flood. 


As  the  flood  recedes,  relict  pools  on  the  bottom-lands  and  along 
the  margins  of  the  permanent  backwaters  are  formed,  in  which  the 
conditions  favoring  sporulation  or  other  means  of  providing  for 
resuscitation  are  to  be  found.  The  emerging  bottom-lands  thus  be- 
come the  seed-bed  for  starting  a  new  cycle  of  diatoms  whenever  flood 
conditions  return.  In  the  river,  on  the  other  hand,  the  conditions 
for  sporulation  are  not  so  favorable,  and  the  current  tends  to  carry 
away  such  resting  stages  as  may  be  formed.  The  observed  facts 
regarding  the  distribution  of  diatoms  and  the  examination  of  the 
conditions  under  which  these  pulses  occur  thus  alike  yield  corrob- 
oration  of  the  view  that  floods  are  potent  factors  in  determining  the 
occurrence  of  diatoms  in  fluviatile  waters,  especially  where  back  waters 
are  extensive. 

The  nature  of  the  action  of  floods  is  in  some  respects  similar  to 
that  of  the  overturning  of  the  water  which  occurs  in  lakes  when  the 
point  of  maximum  density,  39.2°,  is  passed  in  either  direction.  In  lakes 
of  some  depth  the  vertical  circulation  of  so  large  a  volume  of  water 
results  in  a  stirring  up  of  the  bottom  deposits  containing  the  resting 
stages  of  diatoms,  so  that  they  are  brought  again  into  increased 
light  and  to  better  aeration.  Whipple  ('94)  has  emphasized  the 
importance  of  this  overturning  in  starting  the  growth  of  diatoms. 
In  our  shallow  waters  this  physical  phenomenon  is  of  less  impor- 
tance than  in  the  deeper  waters  of  the  lake  or  reservoir.  The  vol- 
ume in  circulation  is  smaller,  though  some  compensation  for  this 
may  exist  in  the  possibility  of  repeated  over  turnings  with  fluctua- 
tions in  temperatures  at  the  critical  stage.  The  existence  of  cur- 
rents, the  movements  of  fish,  and  the  roiling  effect  of  strong  and 
long-continued  winds  upon  our  shallow  backwaters,  combined  with 
the  fact  that  much  of  the  seed-bed  area  of  overflow  is  dry  land  at 
the  time  of  the  autumnal  overturning,  all  serve  to  minimize  the 
effect  of  this  overturning  in  our  waters  upon  the  growth  of  diatoms 
in  the  plankton.  The  spring  overturning  occurs  early  in  March, 
and  in  1896, 1898,  and  1899  a  slight  pulse  not  exceeding  an  increase  of 
100  per  cent,  follows  the  overturning  within  an  interval  of  a  fortnight. 
The  vernal  pulse  is  about  two  months  later  than  the  overturning, 
and  the  relation  of  this  to  the  overturning  does  not  seem  to  be  inti- 
mate. The  autumnal  overturning  occurs  towards  the  middle  or  end 
of  November,  and  in  1895,  1896,  and  1898  the  hiemal  pulse  of 
December  follows  close  upon  it,  within  two,  or  at  most  three,  weeks. 


41 

The  relation  is  here  more  apparent,  but  the  resulting  pulse  is  no 
larger  than  those  following  upon  floods  during  summer,  and  but 
little  larger  than  the  ones  which  precede  it  in  the  autumn.  The 
effect  of  this  overturning  upon  the  plankton  of  the  Illinois  River 
may  thus  be  detected,  though  it  is  here  of  less  importance  than  in 
lakes  and  reservoirs  since  it  is  overshadowed  or  replaced  by  other 
and  more  potent  factors. 

The  relation  of  the  seasonal  distribution  of  the  diatoms  to  that 
of  the  total  plankton  is  not  readily  unraveled.  The  latter  is  the 
resultant  of  a  most  complex  series  of  factors,  whose  number  and 
relative  potency  are  subject  to  constant  change  and  readjustment 
in  the  unstable  environment  of  the  stream.  It  is  the  biological 
expression  of  the  state  of  tension  among  these  various  factors  which 
for  the  moment  exists.  Of  these  factors  the  diatoms  are  but  one, 
though  an  important  one,  in  the  food  cycle  and  ecology  of  the 
plankton.  The  volumetric  determinations  in.  the  diagram  (p. 
37)  do  not  give"  the  true  seasonal  distribution  of  the  total  plankton 
owing  to  the  escape  of  an  unknown  quantity  through  the  meshes  of 
the  silk  net.  They  represent  more  truly  that  of  the  animal  plank- 
ton than  that  of  the  phytoplankton.  A  comparison  of  the  seasonal 
distribution  of  the  diatoms  and  total  plankton  may  serve,  in  spite 
of  the  errors  involved  in  the  volumetric  determinations  and  the 
disparity  of  individuals  among  the  diatoms,  to  throw  some  light  on 
the  effect  of  the  fluctuations  of  the  latter  upon  the  movement  in  the 
volume  of  plankton.  A  close  comparison  of  the  two  seasonal  curves 
reveals  the  fact  that  the  diatom  curve  is  not  identical  with  the  vol- 
umetric curve.  It  is  true  that  the  double  vernal  (April-May)  pulse 
of  diatoms  coincides  in  location  with  the  vernal  volumetric  pulse. 
This  is  also  true  of  the  pulses  of  June  14  and  July  19.  The  crest  of 
the  volumetric  vernal  pulse  is,  however,  lodged  between  the  double 
apices  of  the  diatom  curve,  and  all  the  subsequent  volumetric 
pulses  from  July  on  lie  in  depressions  of  the  diatom  curve,  and  vice 
versa.  It  is  apparent  at  once  on  examination  of  our  planktons  that 
the  catches  of  the  silk  net  are  from  the  volumetric  standpoint 
largely,  indeed  overwhelmingly,  of  animal  origin.  These  volu- 
metric pulses  are  as  a  rule  largely  pulses  of  the  zooplankton.  It  is 
therefore  to  be  expected  that  the  diatoms  would  decrease  at  such 
times,  since  they  form  the  food  of  many  Entomostraca  and  not  a  few 
Rotifera.  The  appearance  of  the  diatom  pulses  before  or  after  the 


42 

volumetric  (animal)  pulse  may  therefore  in  a  measure  present  the 
wavering  tendency  to  establish  an  equilibrium  between  these  two 
elements  of  the  plankton.  The  presence  of  an  abundant  animal 
plankton  may  therefore  be  a  cause  of  some  of  the  minimum  periods 
between  diatom  pulses.  Other  causes,  such  as  decline  of  food  ele- 
ments, may  also  arise,  but  in  our  waters  the  nitrates  at  least  rarely 
ever  reach  a  level  where  an  unutilized  margin  capable  of  support- 
ing a  large  diatom  population  is  not  still  present.  Data  concerning 
other  food  elements  are  not  at  hand,  but  their  paucity  in  water 
derived  from  such  varied  sources  and  so  liberally  fertilized  by 
organic  wastes  seems  improbable.  There  is  also  the  further  possi- 
bility— and,  indeed,  from  the  data  in  hand  the  probability — of  the 
existence  among  diatoms  of  reproductive  cycles,  interrupted  by 
resting  periods.  The  available  data  do  not,  however,  throw  any 
light  upon  the  nature  of  this  internal  factor  or  the  cause  for  the 
running  down  of  the  energy  of  reproduction,  and  but  little  upon  the 
operation  of  environmental  factors  which  stimulate  anew  the 
process  of  reproduction. 

The  seasonal  distribution  of  the  diatoms  as  a  whole,  and  that  of 
individual  species  also,  offer  repeated  instances  of  recurrent  pulses 
at  intervals  approximating  four  weeks — the  lunar  month.  In  1898 
thirteen  such  pulses  can  be  detected.  These  often  correspond 
roughly  to  minor  flood  intervals,  but  not  always  so,  for  occasionally 
two  pulses  occur  on  the  decline  of  a  single  flood.  Similar  appear- 
ances may  be  traced  in  other  years,  when  collections  were  frequent 
enough  to  exhibit  minor  pulses.  They  are,  however,  in  all  cases 
quite  irregular,  and  exceptions  are  frequent. 

That  cosmic  factors  may  indirectly,  through  immediately  environ- 
ing factors,  affect  the  reproductive  phenomena  of  pelagic  organisms 
has  been  suggested  by  the  work  of  Kramer  ('97),  Mayer  ('00),  and 
Friedlander  ('01)  in  the  case  of  the  "Palolo"  worm,  a  coral-reef 
annelid  whose  seasonal  swarming  for  reproductive  purposes  occurs 
at  somewhat  definite  lunar  intervals. 

While  the  data  concerning  the  seasonal  distribution  of  diatoms 
in  the  Illinois  River  may  serve  to  suggest  the  operation  of  an  enig- 
matic cosmic  factor,  I  wish  distinctly  to  state  that  in  my  opinion 
they  are  wholly  inadequate  to  establish  either  its  presence  or  its 
potency.  It  is  much  more  probable  that  we  have  to  deal  merely 
with  some  matter  of  food  relations  between  the  plants  and  animals 


43 

of  the  plankton,  and  perhaps  with  the  result  of  increased  photo- 
synthesis in  periods  of  lunar  illumination,  which  tends  to  establish  the 
limits  of  the  pulses. 

The  number  of  forms  of  diatoms  noted  in  our  records  in  the 
plankton  of  the  Illinois  River  is  thirty-one.  This  number  could  be 
greatly  increased  by  the  inclusion  of  the  many  adventitious  species 
which  flood-waters  bring  into  the  plankton  and  by  the  addition  of 
rarer  limnetic  species.  Of  these  thirty-one  at  least  twelve  are 
eulimnetic,  while  the  others  are  in  the  main  adventitious.  There 
are  no  species  among  them  peculiar  to  the  potamoplankton,  and 
the  dominant  forms  here  are  also  abundant  in  the  fresh-water  plank- 
ton of  our  own  Great  Lakes  and  of  European  streams  and  lakes, 
barring  a  few  mooted  points  of  specific  identity. 

The  limnetic  species  are  fourteen  in  number,  viz. :  Asterionella 
formosa,  A.  gracillima,  Cydotella  kuetzingiana,  Diatoma  elongatum 
var.  tenue,  Fragilaria  crotonensis,  F.  virescens,  Melosira  .  granulata 
var.  spinosa,  M.  variant,  Meridian  circulare,  Rhizosolenia  eriensis, 
Stephanodiscus  magaroz,  Synedra  acus,  S.  acus  var.  delicatissima, 
and  Tabellaria  fene  strata.  Of  these  limnetic  forms  the  more  impor- 
tant ones  are  Asterionella  gracillima,  Cydotella,  Fragilaria  virescens, 
Melosira  granulata  var.  spinosa,  and  Synedra  acus  and  its  varieties. 
The  absence  or  small  number  of  certain  limnetic  species  is  notice- 
able. These  are  several  species  of  Tabellaria  and  Attheya.  On  ac- 
count of  the  abundance  of  silt  and  the  transparency  of  Attheya  it 
may  have  been  overlooked.  It  has  hitherto  been  reported  from 
waters  much  nearer  the  sea,  and  this  coupled  with  its  affinities  to 
marine  diatoms  may  explain  its  absence  in  our  waters. 

The  remainder  of  the  forms  are  adventitious,  or  largely  so,  and 
with  the  exception  of  the  species  of  Navicula  they  have  little  effect 
upon  the  ecology  or  quantity  of  the  potamoplankton. 

DISCUSSION    OF    SPECIES    OF    BACILLARIACE^I. 

Asterionella  formosa  Hassall. — Average  number  of  individual 
cells,  960.  Average  size  of  colony,  4.8  cells.  Recorded  only  in 
November,  December,  and  from  February  through  April,  and  never 
in  large  numbers.  The  greatest  pulse  attained  at  any  time  cul- 
minated on  March  30,  1896,  at  54,540.  Aside  from  an  isolated 
occurrence  on  June  27,  1896,  no  individuals  were  recorded  at  tem- 
peratures above  48°,  and  three  fourths  of  the  occurrences  are  at 


44 

temperatures  below  40°.  The  data  .are  insufficient  to  trace  the 
pulses  satisfactorily.  This  species  is  distinguished  with  difficulty 
from  A.  gracillima,  and  may  include  only  old,  and  in  our  planktons 
often  heavily  incrusted,  individuals;  or  it  may  be  only  a  low-tem- 
perature variety  of  the  species  above  named,  which  in  the  grand 
total  of  all  our  collections  outnumbers  it  ten  thousand  to  one. 

Asterionella  gracillima  Heib. — Average .  number  of  individual 
cells,  28,860,160.  In  1897  the  species  was  only  one  third  as  abun- 
dant, a  contrast  which  finds  its  explanation  in  the  fact  that  the 
June  rise  of  that  year  (Pt.  I.,  PI.  XI.)  did  not  reach  the  stage  of 
overflow,  and  a  June  pulse  is  absent  in  the  collections  of  that  year. 
The  seasonal  distribution  of  this  organism  is  one  of  the  best-defined 
and  most  striking  of  all  the  components  of  the  river  plankton.  It  is 
peculiar  in  the  fact  that  it  appears  in  numbers  only  during  spring  and 
the  beginning  of  summer,  and  in  the  absence  of  any  autumnal  pulse 
upon  the  return  of  the  temperatures  in  which  the  spring  pulse  ap- 
peared. This  species  was  recorded  in  every  month  of  the  year  but 
October,  but  always  in  small  numbers  after  July  1.  In  1894,  collec- 
tions were  not  commenced  until  after  the  time  of  the  spring  pulse. 
In  1895  the  spring  collections  were  few,  and  at  intervals  so  great  as  to 
•preclude  the  detection  of  the  full  course  of  the  spring  pulse.  The 
maximum  number  in  the  collections  'of  that  year  appears  April  9  at 
1,203,100  and  falls  to  445,995  on  April  29 — which  is  approximately 
the  time  of  the  maximum  of  subsequent  years.  This  was  a  year  of 
unusually  low  water  during  the  spring,  and  overflow  stage  was  at  no 
time  reached  (Pt.  I.,  PL  IX.),  which  may  account  for  the  apparent 
suppression  of  the  spring  pulse.  The  species  does  not  reappear  in  the 
collections  of  that  year  until  December,  but  it  continues  in  small 
numbers  (less  than  5,000  per  m.3)  until  the  end  of  March,  1896,  when 
there  is  a  rapid  increase  which  culminates  April  24  at  26,281,400.  It 
disappears  entirely  from  the  records  at  the  end  of  a  fortnight,  and  save 
for  a  single  entry  in  June  and  two  in  September  it  does  not  again 
appearin  1896.  In  1897  the  culmination  of  thespring  pulse  occurs  April 
27  at  324, 633, 600 — three  hundred-fold  larger  than  in  the  previous 
year.  There  is  a  normal  March  flood  (Pt.  I.,  PI.  XL),  on  the  declining 
stages  of  which  this  pulse  appears.  With  the  close  of  June  the 
species  disappears  from  the  records.  The  June  rise  does  not  reach 
the  stage  of  overflow,  and  the  scanty  records  show  but  this  single 
pulse  throughout  the  year.  Beyond  a  single  entry  in  August  and  in 


45 

November  the  species  does  not  again  appear  in  the  records  during 
the  year.  In  1898  there  is  an  unusual  midwinter  pulse  on  January 
11  of  146,280,  followed  by  a  decline  and  irregularities  due  to  the  ris- 
ing winter  flood  (Pt.  I.,  PI.  XII.).  At  the  middle  of  March  a  rapid 
increase  ensues,  culminating  April  26  at  891, 648,000  ^on-the  declin- 
ing spring  flood.  A  decline  to  197,683,200  is  found  at  the  close  of  a 
week,  and  it  is  accelerated  by  the  secondary  spring  flood,  which 
attains  the  overflow  stage  of  15  feet  in  the  closing  days  of  May 
(Pt.  I.,  PI.  XII.).  With  the  decline  of  this  flood  in  June  a  second 
pulse  appears,  increasing  from  15,080  on  May  26  to  336,194,880  on 
June  14,  and  at  the  end  of  three  weeks  the  species  practically  dis- 
appears from  the  plankton.  A  few  scattered  entries  appear  during 
the  summer  and  fall,  and  a  minor  pulse  of  10,500  appears  on  Decem- 
ber 20,  followed  by  a  decline  in  the  next  month. 

This  species  in  our  waters  exhibits  a  well-defined  vernal  pulse 
towards  the  end  of  April  at  about  60°,  but  no  autumnal  pulse 
appears  when  this  temperature  recurs.  , There  is  a  slight  indica- 
tion of  a  minor  midwinter  pulse  at  the  minimum  temperatures 
of  the  year.  This  occurrence  of  a  midwinter  pulse  was  noted 
by  Whipple  and  Jackson  ('99)  in  the  reservoirs  of  the  Brooklyn 
water- works,  and  in  the  same  paper  its  seasonal  distribution  in 
Fresh  Pond,  Lake  Cochituate,  and  Wenham  Lake,  Massachusetts, 
is  given  for  the  years  1890-97,  in  the  majority  of  which  a  mid- 
winter pulse  commensurate  in  magnitude  with  the  vernal  pulse  is  to 
be  found.  Autumnal  pulses  are  of  infrequent  occurrence,  the  vernal 
pulse  being  the  most  frequent  but  not  constant.  In  European 
waters  no  such  long-continued  examination  of  the  seasonal  distribu- 
tion of  this  organism  has  as  yet  been  reported.  Apstein  ('96)  finds 
two  pulses  per  year  in  Ploner  See — in  May  and  the  last  of  July ;  and 
two  in  Dobersdorfer  See,  one  in  April  and  one  in  October,  separated 
by  midsummer  and  midwinter  minima.  Lauterborn  ('93)  finds  that 
this  species  in  the  "  Altwasser"  of  the  Rhine  attains  its  maximum  in 
June  and  again  increases  in  October.  In  the  backwaters  of  the  Elbe, 
Schorler  ('00)  reports  Astenonella  as  abundant  in  April,  June,  July, 
and  October,  but  refers  the  organisms  to  the  preceding  species.  The 
existence  of  the  vernal  pulse  only  in  our  waters  is  thus  somewhat 
unique,  and  the  cause  of  the  phenomenon  probably  lies  in  some 
environmental  conditions,  perhaps  in  our  peculiar  bacterial  and 
sewage  contamination  of  the  autumn.  Our  vernal  pulses  appear  on 


46 

declining  floods  about  the  end  of  April  at  about  60°.  It  can  not  be 
temperature  which  limits  the  occurrence  of  the  species,  for  this 
apparent  optimum  recurs  again  in  October.  This  is  the  period  of 
declining  nitrates  (Pt.  I.,  PL  XLIII.-XLV.),  but  they  rise  again  in 
the  autumn,  and  in  our  sewage-fed  waters  they  contain  even  in  the 
midsummer  minimum  a  quantity  adequate  to  support  an  abundant 
growth  of  Asterionella.  Whipple  and  Jackson  ('99)  have  found  on 
analysis  that  Asterionella  to  the  number  of  10,000,000,000  per  cubic 
meter  yield  but  .079  parts  per  million  of  organic  nitrogen.  The  nitrates 
in  our  waters  rarely  fall  below. 25  parts  permillion,  which,  with  the  other 
forms  of  nitrogen  that  may  be  available,  would  seem  to  afford  nurture 
not  only  for  Asterionella  but  also  for  competing  organisms.  These 
authors  have  also  found  that  silica  to  the  amount  of  1 . 78  and  manganic 
oxide  to  .03  per  million  are  contained  in  Asterionella  to  the  number 
per  cubic  meter  above  quoted.  As  was  shown  in  Pt.  I.,  p.  234,  the 
silica  is  present  in  great  excess  (26  to  81  parts), and  the  manganic 
oxide,  though  not  reported  in  the  analyses  of  November  waters,  is 
present  on  June  15  to  the  amount  of  .07  parts  per  million — more  than 
double  the  amount  required  to  support  Astenonella  to  a  maximum 
twelve  times  as  great  as  any  recorded  in  our  plankton  collections. 
This  also  occurs  at  a  season  when  Asterionella  is  usually  declining 
rapidly  in  numbers.  Such  chemical  data  as  are  available  thus  afford 
us  no  explanation  of  the  limitation  of  Asterionella  in  our  waters  to 
the  vernal  pulse  alone. 

Some  evidence  bearing  on  a  factor  which  may  be  operative  in 
producing  this  phenomenon  is  to  be  found  in  the  hydrographic  con- 
ditions attending  the  vernal  pulse.  As  previously  noted,  this 
appears,  each  year  with  the  decline  of  the  spring  flood.  A  repetition 
of  the  overflow  in  1898  at  the  end  of  May  brought  with  it  a  repetition 
of  the  vernal  pulse  of  Asterionella  in  early  June.  With  the  decline 
of  the  flood  the  backwaters  make  their  major  contribution  to  the 
channel  plankton,  and  it  is  during  this  period  that  Asterionella 
reaches  its  maximum  and  also  declines.  If  the  spring  flood  is  sup- 
pressed, as  in  1895  and  1896,  the  spring  pulse  of  Asterionella  is  cor- 
respondingly feeble.  The  environmental  conditions  are  thus  more 
favorable  in  the  impounded  backwaters  than  in  the  main  stream. 
Whipple  and  Jackson  ('99)  have  noted  in  frustules  of  this  diatom 
the  appearance  of  structures  which  they  interpret  as  spores.  If  these 
are  spores,  and  if  the  sedimentation  of  spore-bearing  frustules  occurs 


47 

extensively  in  the  relict  pools  of  the  emerging  bottom-lands,  a  seed- 
bed for  re-stocking  the  waters  of  overflow  is  formed  with  each  declin- 
ing flood,  and  this  seed-bed  becomes  potent  only  when  floods  return. 
The  absence  of  an  autumnal  overflow  and  the  minor  part  that  the 
autumnal  overturning  plays  in  our  shallow  waters  whm  39.2°  is 
passed, may  alike  tend  to  suppress  here  the  autumnal  or  midwinter 
pulses  which  occur  elsewhere  in  deeper  water. 

The  occurrence  of  the  vernal  pulse  of  Asterionella  in  the  last  days 
of  April  brings  it  into  close  relation  with  the  major  volumetric  pulse 
of  the  year  (Pt.  I.,  PL  IX.-XIL).  It  is  not  only  an  important  con- 
stituent of  this  spring  maximum,  but  it  is  one  of  the  most  prominent 
primal  sources  of  food  of  the  Entomostraca — Bosmina,  Daphnia, 
Cyclops,  and  Diaptomus,  all  of  which  exhibit  an  increase  in  numbers 
at  this  period.  It  shares  with  Cyclotella  the  claim  to  the  first  place 
quantitatively  among  the  synthetic  organisms  upon  which  the 
early  spring  plankton  depends  for  its  development. 

Our  records  are  all  based  upon  the  catches  of  the  silk  net,  through 
whose  meshes  the  isolated  cells  of  Asterionella  readily  escape.  Filter- 
paper  catches  give  much  higher  numbers  except  during  the  period  of 
maximum,  when  the  numbers  by  the  two  methods  do  not  materially 
differ.  This  seems  to  be  d.ue  to  the  fact  that  isolated  cells  are  rela- 
tively much  more  abundant  after  the  maxima  than  they  are  be- 
fore them,  and  especially  at  the  time  of  their  appearance.  These 
diatoms  form  arcs,  circles,  or  whorls,  of  a  varying  number  of  cells. 
During  the  vernal  pulses  of  1898  the  average  number  in  these  clus- 
ters in  the  middle  of  March  was  three  or  four,  and  at  the  time  of  the 
maximum  on  April  26  it  rose  to  five  or  six,  often  reaching  sixteen  or 
more.  A  fortnight  after  this  maximum  the  average  fell  to  1.4,  rising 
again  with  the  second  pulse,  on  June  14,  to  8.4,  and  declining  in  three 
weeks,  with  the  fading  out  of  the  pulse,  to  1.2. 

Asterionella  is  frequently  infested  with  great  numbers  of  a  minute 
craspemonad  flagellate  protozoan  which  appears  in  thick-set  rows 
upon  the  ray-like  cells,  a  single  cell  sometimes  bearing  a  score  of 
these  organisms.  This  diatom  exhibits  considerable  variation  in  size 
and  proportions.  The  longer  and  more  slender  cells  appear  at  the 
times  of  the  maxima. 

Cocconeis  communis  Heib.* — Average  number,  520,000,  but  more 
than  three  times  as  abundant  in  1897.  This  diatom  occurs  some- 
what irregularly  in  the  filter-paper  collections,  and  has  been  recorded 


48 

in  every  month  of  the  year.  It  is  somewhat  more  prevalent  in 
spring  and  autumn,  and  there  are  indications  of  a  vernal  pulse  in 
May  and  an  autumnal  one  in  September,  separated  by  prolonged 
midsummer  and  midwinter  minima.  Vernal  pulses  appear  in  1897 
on  June  28  at  14,400,000,  and  in  1898  on  May  17  at  7,200,000.  Autum- 
nal pulses  occur  in  1896  on  September  16  at  2,700,000;  in  1897  on 
September  29  at  10,800,000;  and  in  1898  on  September  13  at 
5,400,000.  The  optimum  temperatures  lie  between  60°  and  75°,  the 
autumnal  pulse  appearing  in  higher  temperatures  than  the  vernal  as 
a  rule.  This  diatom  is  reported  as  often  epiphytic  upon  algae,  and 
it  may  be  wholly  adventitious  in  the  plankton.  There  is  nothing, 
however,  in  the  curve  of  its  distribution  to  corroborate  this  view. 

Cydotella  kuetzingiana  Thw.* — Average  number  243,659,615, 
but  slightly  more  abundant  in  the  preceding  year.  This  is  one  of  the 
smallest  as  well  as  one  of  the  most  abundant  of  all  the  diatoms  of  the 
river  plankton.  It  readily  escapes  through  the  meshes  of  the  silk 
net,  and  plankton  collections  made  by  this  means  give  no  adequate 
conception  of  its  prevalence  or  importance  in  the  ecology  of  the 
plankton.  It  appears  in  every  month  in  the  year  and  in  practically 
all  of  our  collections,  and  is  thus  a  perennial  planktont.  There  is  a 
considerable  variation  in  size  among  the  individuals  in  the  plankton, 
but  the  greater  number  lie  near  the  smaller  rather  than  the  larger 
limits.  It  may  be  that  several  species  have  been  combined  in  the 
enumeration. 

The  fluctuations  in  the  seasonal  distribution  of  this  diatom  are 
considerable,  and  pulses  occur  at  all  seasons  of  the  year.  The  vernal 
pulse  is,  however,  preeminent,  and  is  not  approached  in  magnitude 
by  those  of  any  other  season  of  the  year.  In  1897  this  pulse  culmi- 
nates at  5,724,000,000  on  April  27,  and  in  1898  on  April  26  at  2,880,- 
000,000.  Throughout  the  summer  and  autumn  in  both  years  there 
is  a  series  of  minor  pulses  at  intervals  of  two  to  eight  weeks.  In  1897 
an  autumnal  pulse  of  223,200,000  appears  on  September  29,  and 
though  not  of  greater  magnitude  than  two  previous  summer  pulses, 
it  does  surpass  anything  prior  to  the  pulse  of  the  following  spring. 
In  1898  there  are  seven  pulses  during  the  summer  and  fall,  culmi- 
nating as  follows:  on  May  10  at  2, 668, 000,000;  on  June  28  at  291,- 
000,000;  on  July  19  at  561,600,000;  on  August  9  at  401,400,000;  on 
August  23  at  122,400,000 ;  on  September  6  at  1 15,200,000 ;  on  Septem- 


49 

ber  27  at  57,600,000;  on  October  25  at  25,200,000;  and  in  December 
a  pulse  well  sustained  throughout  the  month  culminates  on  the  15th 
at  414,000,000. 

The  temperature  optimum  appears  to  be  about  60°,  though  its 
return  in  the  autumn  does  not  induce  a  development  comparable 
with  that  of  the  closing  days  of  April.  The  midsummer  pulses  and 
that  of  December  show  that  other  causes  than  temperature  are 
operative  in  regulating  the  occurrence  of  this  organism. 

The  appearance  of  the  vernal  pulse  of  Cyclotella  at  the  time  of 
the  volumetric  maximum  (Pt.  I.,  PL  IX.-XII.)  in  April-May  sug- 
gests its  function  as  one  of  the  primal  sources  of  food  for  the  animal 
components  of  that  plankton.  The  plates  are  based  on  collections 
of  the  silk  net,  and  Cyclotella  constitutes  an  insignificant  part  of  the 
volumetric  total  there  graphically  presented,  since  it  is  so  small  that 
it  escapes  readily  through  the  silk. 

Cymatopleura  solea  (Breb.)  W.  Sm.* — Average  number,  2,115 
(silk,  1,292),  but  slightly  more  abundant  in  1897.  Isolated  occur- 
rences in  small  numbers  appear  during  the  colder  months,  generally 
below  60°,  though  several  individuals  appear  in  summer  records. 
This  is  apparently  an  adventitious  planktont,  whose  presence  is 
often  due  to  flood  waters. 

Diatoma  elongation  var.  tenue  Van  Heurck.* — Average  number, 
2,471,923.  This  is  a  perennial  limnetic  diatom  occurring  in  every 
month  of  the  year  and  in  the  majority  of  our  collections.  It  is  but 
sparingly  present  during  midsummer.  There  are  well-defined 
vernal  pulses  in  1897  on  May  25  of  50,400,000,  and  in  1898  on  May 
3  of  18,000,000.  A  second  large  pulse  appears  on  the  approach  of 
winter,  in  1897,  on  November  15,  culminating  at  2,700,000,  and  in 
1898,  on  November  22,  at  9,000,000.  In  the  silk  collections  of  1895 
and  1896  pulses  also  appear  in  the  last  days  of  April  and  in  Novem- 
ber or  December.  The  records  thus  indicate  a  decided  preference 
of  the  species  for  temperatures  below  70°  and  the  possibility  of 
rapid  development  in  midwinter — as  in  1895,  during  a  fortnight  of 
minimum  temperatures  (32°  +  ),  culminating  at  53,424  (silk)  Decem- 
ber 18.  The  vernal  pulses  coincide  approximately  with  the  volu- 
metric maximum,  and  the  December  pulse  of  1895  attends  an 
unusual  winter  development  of  the  plankton  (Pt.  I.,  PI.  IX.  and 
Table  III.). 


50 

Diatoma  vulgare  Bory  occurred  sparingly  at  irregular  intervals, 
and  is  apparently  an  adventitious  species  in  the  plankton. 

Encyonema  prostratum  (Berk.)  Ralfs  appears  a  few  times  during 
the  summer  months,  and  is  evidently  adventitious,  as  is  also  the 
still  rarer  Epithemia  turgida  Kiitz. 

Fragilaria  crotonensis  (Edw.)  Kitton. — Average  number  of  cells, 
2.1.  This  limnetic  diatom  is  much  less  abundant  in  our  waters 
than  the  following  species.  In  1898  it  appeared  in  February,  and 
increased  from  19,200  on  April  19,  to  14,469,120  on  May  10,  dis- 
appearing entirely  from  the  records  after  May  17.  Such  meteoric 
pulses  were  not  detected  in  previous  years,  when  only  scattered 
entries  in  April,  May,  and  December  were  recorded.  The  number 
of  cells  in  the  filaments  is  very  much  less  than  in  F.  virescens,  aver- 
aging but  14  to  its  108.  Its  optimum  temperature  lies  about  60°, 
and  its  vernal  pulse  occurs  immediately  after  the  volumetric  maxi- 
mum (Pt.  I.,  PI.  XII.)  and  upon  the  same  date  with  that  of  F.  vi- 
rescens. It  seems  to  be  predominantly  a  vernal  planktont  in  our 
waters.  In  German  lakes  Apstein  ('96)  finds  maxima  as  late  as 
June-July,  but  always,  it  seems,  at  temperatures  below  70°. 

Fragilaria  virescens  Ralfs. — Average  number,  73.1.  Apparently 
ten  times  more  abundant  than  in  1897,  as  a  result  possibly  of  the 
absence  of  collections  during  the  period  of  the  vernal  maximum  in 
that  year.  This  is  a  perennial  organism,  with  two  well-defined 
pulses  ;  a  vernal  one  in  April-May  and  another  in  November- 
December.  The  uniformity  with  which  these  pulses  appeared  in 
1895-1898  is  very  striking  when  one  considers  the  unstable  environ- 
ment in  which  the  pulses  occur.  In  1894  the  species  is  not  present 
in  numbers  in  any  of  the  scattered  collections  of  the  year.  In  1895 
the  vernal  pulse  is  indicated  in  the  collection  of  April  29  (2,754,675), 
after  which  the  species  disappears  until  September,  increasing — 
with  a  temporary  backset  by  the  December  flood  (Pt.  I.,  PI.  IX.)— 
to  a  second  culmination  December  30  at  282, 225.  After  a  minimum 
in  January,  1896,  the  numbers  increase,  with  minor  fluctuations,  to 
a  vernal  maximum  of  76,224,000  on  April  24,  followed  by  a  mini- 
mum period  from  May  18  to  the  following  November.  The  winter 
pulse  again  appears  in  December,  culminating  on  the  3d  at  867,048. 
In  1897  the  vernal  pulse  seems  to  culminate  somewhat  later  than 
usual,  though  the  interval  of  collection  is  too  great  to  follow  its  full 
course.  The  maximum  appears  on  May  25  at  3,549,600,  after 


51 

which  the  species  dwindles  away  and  disappears  in  August  to  return 
early  in  November.  The  winter  pulse  culminates  December  14  at 
8,159,250,  at  a  break  in  the  ice  blockade.  In  1898  the  winter  mini- 
mum continues  into  April,  and  the  vernal  pulse  appears  May  10  at 
253,960,000,  rising  with  rocket-like  suddenness  from  390,000  of  the 
previous  week,  and  declining  the  week  following  to  4,110,400.  The 
decline  to  the  summer  minimum  is  prolonged  into  July,  and  the 
species  does  not  reappear  until  October.  The  winter  pulse  begins 
earlier  than  usual,  on  November  1,  and  is  well  sustained  through 
the  month,  culminating  on  the  29th  at  2,254,000.  The  winter  mini- 
mum which  follows,  does  not  reach  the  low  levels  of  that  of  summer. 
This  species  has  thus  a  characteristic  distribution,  the  analysis 
of  which  is  by  no  means  simple.  The  contrast  between  the  summer 
and  winter  minimum  may  be  due  to  the  low  nitrates  of  the  summer 
and  the  larger  amount  in  the  winter  (Pt.  I.,  PI.  XLIII.-XLV.), 
which  favor  a  proportionate  development  of  this  diatom,  though 
not  every  species  shows  this  response.  The  two  minima  separate 
the  seasonal  occurrences  of  this  species  into  two  periods  of  growth ; 
a  vernal,  from  March  to  June,  and  a  hiemal,  from  October  to  Janu- 
ary, the  limits  and  relative  development  of  each  being  somewhat 
variable  from  year  to  year.  The  temperatures  of  the  two  periods 
differ.  Both  are  times  of  rapid  change, — of  rise  and  fall  respectively, — 
and  the  culminations  of  the  periods  of  growth  lie  at  widely  sepa- 
rated temperatures.  The  vernal  pulses  in  1896  and  1898 — in  which 
years  collections  were  frequent  enough  to  locate  them  with  some 
degree  of  accuracy — appear  at  72°  (April.  24)  and  61°  (May  10) 
respectively,  and  in  every  year  the  vernal  pulse  appears  during  a 
period  of  rapid  change.  The  hiemal  pulse,  on  the  other  hand,  cul- 
minates in  each  year  after  the  winter  minimum  approaching  32°  has 
been  reached,  and  in  two  years  during  the  ice  blockade.  Tempera- 
ture within  these  limits  seems  not  to  be  a  determining  factor  in  the 
pulses  of  this  organism.  The  nitrates  (Pt.  I.,  PI.  XLIII.-XLV.) 
have  been  uniformly  high  (above  2  parts  per  million)  whenever  the 
pulses  occurred.  In  1898  they  decline  abruptly  (Pt.  I.,  PI.  XLV) 
and  remain  at  a  low  level  throughout  December,  and  in  this  month, 
when  usually  Fragilaria  attains  its  hiemal  maximum,  we  find  it 
dropping  to  the  unusual  minimum  of  20,000.  The  pulse  which 
began  in  November  is  cut  off  apparently  by  this  unusual  decline  in 
nitrates.  Abundance  in  nitrates  is  not,  however,  in  itself  sufficient 

(5) 


52 

to  cause  a  pulse  of  development  of  Fragilana,  for  nitrates  are 
abundant  when  the  diatom  declines  and  is  at  its  minimum.  It 
does  not  seem  possible  to  find  in  the  unstable  environment  of  this 
organism  any  external  factor  which  shows  a  causal  connection  with 
its  periods  of  growth. 

Apstein  ('96)  found  that  this  diatom  reached  its  major  pulse 
in  March  and  April  in  Dobersdorfer  See,  and  a  minor  one  in 
November. 

The  cells  of  this  diatom  form  long  twisted  bands,  visible  to  the 
unaided  eye.  They  reach  a  much  greater  length  in  this  species 
than  in  the  preceding  one,  and  are  longest  during  the  height  of  the 
growing  period,  decreasing  rapidly  in  length  as  it  declines.  The 
average  number  of  cells  in  a  ribbon  at  the  time  of  the  maximum  lies 
between  150  and  200,  and  at  other  times  is  usually  below  100  and 
often  below  2  5 . 

The  vernal  pulse  of  this  species  coincides  with  that  of  F.  croto- 
nensis,  and  appears  either  with  or  just  after  the  volumetric  pulse. 
The  December  pulses  may  in  part  serve  as  primal  food  sources  for 
the  fairly  constant  minor  volumetric  pulse  of  December. 

Gomphonema  constrictuvn  Ehrbg.* — Average  number,  501,923. 
This  species  appears  irregularly,  with  a  predominance  of  occur- 
rences in  May  and  November,  and  is  apparently  adventitious. 

Melosira  granulata  (Ehrbg.)  Ralfs  var.  spinosa  Schroder. 
—Average  number  of  cells,  1,181,125  (filter-paper,  34,762,365). 
In  1897  it  was  more  than  five  times  as  abundant.  In  the 
filter-paper  collections  as  a  whole  it  is  about  fifty  times  as  abun- 
dant as  in  those  of  the.  silk  net.  A  much  greater  proportion 
of  single  cells  and  short  filaments  occurs  in  the  latter  collections, 
since  the  longer  filaments  are  the  more  readily  retained  by  the  silk. 
In  the  discussion  which  follows,  the  data  from  the  silk  collections 
will  be  used,  since  they  cover  the  whole  period.  The  data  from 
the  filter-paper  collections  indicate  very  nearly  the  same  seasonal 
routine,  and  the  differences  between  the  results  by  the  two  methods 
lie  in  the  proportions  of  the  numbers  rather  than  in  the  direction  of 
movement  in  the  fluctuations.  The  pictures  of  the  seasonal 
changes  in  occurrence  of  the  diatom  given  by  the  two  methods  are 
essentially  alike  aside  from  greater  irregularity  during  minimum 
periods,  resulting  from  the  larger  margin  of  error  in  the  filter-paper 
method  as  I  used  it. 


53 

This  Melosira  is  a  perennial  planktont  in  that  it  occurs  in  every 
month  of  the  year  in  the  river.  Its  appearances  from  December 
to  March  are,  however,  irregular,  and  its  numbers  small.  Its  large 
pulses — above  1,000,000— all  lie  between  May  15  and  October  1, 
with  the  single  exception  of  the  pulse  of  April  24,  1896,  culminating 
at  2,056,400,  in  temperatures  of  72°,  occurring  fully  a  fortnight 
earlier  than  usual.  The  major  pulse  seems  normally  to  occur  in 
June;  at  least  in  1896  and  1898,  when  collections  were  frequent  at 
this  season  of  the  year,  such  pulses  appear  on  the  llth  at  12,940,000 
and  on  the  21st  at  32,114,880.  A  June  pulse  also  appears  in  1895. 
September  pulses  appear  in  1895,  on  the  12th,  at  2,254,182,  and  in 
1898,  on  the  27th,  at  5,499,840.  There  is,  however,  no  well-defined 
vernal  and  autumnal  growth  period,  since  large  pulses  occur  through- 
out the  whole  summer.  The  greatest  pulse  on  record  (111,456,000) 
is  on  July  21,  1897,  and  in  1898  there  are  three  minor  pulses  between 
those  of  June  and  September.  Including  the  major  pulses,  there 
are  in  1895  five,  in  1896  six,  in  1897  five,  and  in  1898  eight,  pulses 
at  intervals  of  two  to  six  weeks  between  May  and  October,  the  ones 
at  either  end  of  the  season  being  often  but  slightly  developed,  the 
remainder  usually  running  from  1,000,000  to  5,000,000. 

This  species  is  predominantly  a  summer  planktont,  and  its 
optimum  temperature  lies  above  70°,  the  greatest  number  recorded 
appearing  at  81°.  This  is  one  of  the  most  abundant  diatoms  of 
the  potamoplankton,  and  in  our  waters  it  attains  its  greatest  de- 
velopment during  the  season  of  the  minimum  occurrence  of  nitrates, 
in  whose  utilization  it  is  quantitatively  an  important  agent.  It 
fills  the  gap  between  the  vernal  and  autumnal  or  hiemal  appear- 
ances of  Asterionella  and  Fragilaria,  thus  providing  a  continuous 
source  of  food  for  the  zooplankton  with  wrhich  it  is  associated.  It 
is,  by  virtue  of  its  numbers,  its  size,  and  its  seasonal  distribution, 
quantitatively  and  ecologically  the  most  important  of  all  the 
diatoms  of  the  plankton  of  the  Illinois  River. 

The  only  factor  in  the  environment  to  which  the  limitation  of  the 
rapid  growth  of  this  species 'to  the  May-October  period  can  be  re- 
ferred is  temperature.  There  are  but  three  instances  in  the  records 
of  Melosira  exceeding  100,000  per  m.3  at  temperatures  below  60°,  and 
one  of  these  is  but  a  few  days  prior  to  the  attainment  of  that  temper- 
ature. It  cannot  be  food  which  deters  its  development  below  this 
point,  since  the  nitrates  at  least  are  then  most  abundant  (Pt.  I.,  PL 


54 

XLIII.-L.).  Other  diatoms,  as  in  the  hiemal  pulse  of  Fragilaria, 
develop  in  numbers  at  temperatures  approaching  32°,  but  not  M. 
granulata  var.  spinosa.  Whipple  ('94)  concludes  from  the  records  of 
examinations  of  potable  waters  in  Massachusetts  that  temperature 
has  possibly  a  slight  influence  on  the  growth  of  diatoms,  but  that  it  is 
of  so  little  importance  that  it  does  not  affect  their  seasonal  distribu- 
tion ;  and,  on  the  other  hand,  that  a  sufficient  supply  of  nitrates  is  one 
of  the  most  important  conditions  for  their  growth.  The  seasonal  dis- 
tribution of  Melosira  was  not  separately  discussed  in  his  paper 
though  included  in  his  general  statements.  In  our  waters  the  data 
at  hand  seem  to  show  conclusively  that  abundance  of  nitrates  is  of  no 
avail  in  the  case  of  Melosira  when  the  temperature  falls  below  60°. 
There  are  times,  therefore,  in  the  case  of  this,  our  most  important 
diatom,  when  temperature  is  more  potent  than  food  as  a  factor  con- 
trolling its  growth. 

Melosira  does  not  appear  in  its  maximum  pulses  at  the  time  of  the 
major  volumetric  pulse  of  the  total  plankton  of  April-May,  nor  do 
its  fluctuations  seem  to  bring  about  directly  any  considerable 
changes  in  the  volume  of  the  plankton.  For  example,  the  extreme 
pulse  of  111  ,456,000  on  July  21,  1897,  occurs  at  the  time  of  a  sudden 
drop  in  the  amount  of  plankton  (Pt.  L,  PL  XL).  The  amount  of 
plankton  on  July  14,  21,  and  30  is  8.16,  0.92,  and  1.05  cm.3  per  m.3, 
and  the  corresponding  numbers  of  Melosira  are  66,528,000,  111,456,- 
000,  and  13,176,000. 

The  diatoms  here  discussed  are  predominantly  of  the  type 
designated  as  var.  spinosa,  marked  by  the  spinous  prolongations 
from  the  valves  at  the  ends  of  the  filaments.  The  cells  of  the  forms 
in  our  plankton  are  proportionately  much  longer,  as  a  rule,  than 
those  figured  by  Schroder  ('97),  usually  attaining  one  and  a  half  to 
two  times  the  length  without  proportional  increase  in  diameter. 
Not  infrequently  in  the  height  of  the  growing  season  much  elongated 
and  curved  cells  and  filaments  are  to  be  found.  In  one  instance  an 
unusual  number  of  filaments  approaching  M.  varians  in  form  though 
still,  of  the  spinous  type  were  found.  It  is  not  improbable  that 
several  so-called  species  of  Melosira  have  been  included  with  this 
variable  species  in  the  enumeration. 

Melosira  is  the  bearer  of  numerous  passive  planktonts,  the  most 
abundant  of  which  is  Bicosceca  lacustris  Clk.  Associated  with  this, 
and  often  on  the  same  filament,  is  the  elegant  little  craspemonad 


55 

Salpingceca  brunnea  Stokes.  Cells  to  which  several  of  these  flagel- 
lates are  attached  very  frequently  exhibit  a  breaking  up  of  the  cell 
contents  into  eight  brownish  masses,  often  of  spore-like  form,  and  it 
is  not  an  uncommon  thing  to  find  such  parasitized  filaments  with 
several  empty  cells.  The  eggs  of  the  rotifer  Diurella  ligris  are  fre- 
quently found  attached  to  the  filaments  of  this  diatom.  The  num- 
ber of  cells  in  the  filaments  in  the  silk  collections  averages  6.4  in  1897, 
and  7  in  1898,  while  in  the  filter-paper  collections  it  averages  3.5  in 
both  years.  The  numbers  per  filament  range  from  1  to  40,  and  the 
filaments  are  wont  to  be  somewhat  longer  during  rapid  growth  than 
in  periods  of  decline  or  minimum. 

Melosira  varians  Ag. — Average  number,  148,626  (filter-paper 
3,455,538).  The  discussion  is  based  upon  silk  catches.  The  species 
was  about  equally  abundant  in  1897  but  much  less  so  in  previous 
years.  This  is  a  perennial  species,  reported  in  every  month  of  the 
year  and  in  most  of  the  collections.  It  exhibits  two  well-defined 
pulses,  a  vernal  one  in  April-May  and  an  autumnal  one  in  September- 
October.  The  reduction  in  the  minimum  intervals  varies  from  sea- 
son to  season  and  from  year  to  year.  It  was  most  pronounced,  al- 
most to  suppression,  in  July  and  August  in  1894,  1895,  and  1896,  and 
in  December-February  in  1896-97  and  1898-99.  In  other  seasons 
the  minimum  falls  to  1,000  to  15,000. 

The  vernal  pulse  (146,916)  appears  in  1895  on  April  29,  in  1896 
(229,235)  on  May  18,  in  1897  (2,419,200)  on  May  25,  and  in  1898 
(3,164,160)  on  May  5.  The  autumnal  pulse  (150,720)  is  found  in  1895 
on  October  30;  in  1896,  on  September  16  at  378,900;  in  1897  there 
are  two  pulses,  one  on  August  30  at  738,000,  and  the  other  on  No- 
vember 15  at  458,800;  and  in  1898  one,  on  October  18  at  348,000. 
The  autumnal  pulses  are  thus  much  smaller  than  the  vernal  ones 
and  exhibit  a  greater  range  in  the  time  of  their  appearance. 

As  in  the  case  of  many  other  organisms  this  diatom  also  exhibits 
the  phenomenon  of  recurrent  minor  pulses  at  intervals  of  a  few 
weeks.  They  range  in  height  from  25,000  to  almost  1,000,000,  and 
are  largest  when  found  in  the  proximity  of  the  major  pulses.  The 
records  are  not  frequent  enough  to  trace  them  in  all  seasons.  They 
appear  in  January  in  1896,  1898,  and  1899;  in  February  in  1898; 
twice  in  March  in  1896;  in  April  in  1896;  twice  in  June  in  1897  and 
again  in  1 898 ;  in  July  in  1897  and  1898 ;  in  August  in  1897  and  1898 ; 


56 

in  September  in  1898;  in  November  in  1896,  1897,  and  1898;  and  in 
December  in  1894. 

The  optimum  temperatures,  omitting  the  pulse  of  August  30, 
1897,  at  80°,  all  lie  below  72°,  averaging  65°  for  the  vernal  pulse 
and  62°  for  the  autumnal.  But  three  pulses  in  all,  exceeding 
100,000,  lie  at  temperatures  above  70°,  and  but  three  below  50°. 
In  the  case  of  this  species  likewise  temperatures  seem  to  be  potent 
factors  in  limiting  its  seasonal  occurrence.  The  fluctuations  in 
nitrates  do  not  seem  to  bear  any  constant  relation  to  its  develop- 
ment. The  midsummer  minimum  of  the  diatom  may  appear,  as 
in  1896,  during  an  abundance  of  nitrates  (0.5  to  3.0  parts  per  mil- 
lion— Pt.  I.,  PI.  XLIII.)  unusual  for  the  season.  On  the  other  hand, 
a  minimum  of  nitrates  (.1  to  .35)  in  August  and  December,  1898, 
coincides  with  a  suppression  of  this  species  in  the  plankton.  Thus 
in  the  presence  of  food,  temperature  seems  to  be  a  determining 
factor  in  the  seasonal  distribution  of  this  organism.  Whipple  ('94) 
expresses  the  opinion  that  the  growth  of  diatoms  occurs  at  those 
seasons  of  the  year  when  the  water  is  in  vertical  circulation ;  that  is, 
when  it  passes  39.2°.  In  our  waters  this  generally  occurs  early  in 
March  and  late  in  November.  In  this  species  the  only  pulses 
which  it  seems  might  exhibit  the  effect  of  this  phenomenon  are 
those  of  December  and  March,  and  neither  of  them  are  in  any  way 
constant  or  prominent.  Neither  of  the  major  pulses,  vernal  nor 
autumnal,  can  be  attributed  to  it.  The  latter  pulse  occurs  prior 
to  the  autumnal  overturning  of  the  water. 

The  vernal  pulse  usually  follows  the  spring  volumetric  maxi- 
mum, and  the  autumnal  one  generally  appears  during  a  volumetric 
minimum.  No  immediate  quantitative  effect  of  this  species  upon 
the  plankton  is  apparent. 

In  European  waters  this  is  a  common  planktont,  and  Apstein 
('96)  reports  vernal  maxima  in  March,  April,  and  May,  and  an 
autumnal  one  of  minor  value  in  November. 

The  number  of  cells  in  the  filaments  varies  from  one  to  sixty, 
and  in  filter-paper  collections  averages  four,  while  in  the  silk 
catches  it  varies  from  seven  to  fifteen  from  year  to  year.  The  fila- 
ments average  somewhat  longer  during  the  periods  of  maximum 
growth,  reaching  twelve  to  twenty-five.  This  species  also  occasion- 
ally bears  the  flagellates  found  upon  M.  granulata  var.  spinosa,  but 
not  in  such  abundance.  It  is  quantitatively  much  less  important 


57 

in  our  plankton  than  that  species,  though  this  does  not  seem  to  be 
the  case  in  some  European  waters. 

Meridian  circulare  Ag.  has  appeared  but  four  times  in  winter 
planktons,  from  December  to  March,  and  seems  to_  be  adventi- 
tious. 

Navicula  iridis  Ehrbg.* — Average  number,  297,307.  Appears 
at  irregular  intervals,  often  with  flood  waters  and  in  the  colder 
months.  It  seems  to  be  adventitious. 

Navicula  spp.* — Average  number,  8,569,038.  About  twice  as 
abundant  in  1897.  Under  this  head  I  have  included  a  number  of 
species  of  Navicula,  and,  possibly,  even  species  of  genera  resembling 
Navicula.  The  individuals  are  all  of  small  size,  and  are  principally  of 
the  type  of  the  smaller  forms  of  N.  brebissonii  Kiitz.  and  N.  gracilis 
Ehrbg.  They  are  quite  abundant  in  collections  from  Quiver  Creek 
and  Spoon  River.  Their  greater  abundance  in  1898  as  compared  with 
1897  may  be  caused  by  the  greater  movement  in  river  levels  in  the 
former  year  (85.6  ft.)  as  compared  with  that  of  the  latter  (55.5  ft.). 
This  feature  of  the  distribution  of  these  forms  suggests  that  they 
are  adventitious  in  the  plankton.  This  view  is  further  supported 
by  the  fact  that  some,  though  not  all,  of  their  apparent  pulses 
appear  with  flood  waters;  for  example,  the  pulse  of  64,000,000  on 
May  17,  1898.  There  are  indications,  independent  of  floods,  of 
pulses  in  April-May  and  November-December,  which  may,  how- 
ever, be  simply  reflections  of  pulses  in  the  normal  habitat  of  these 
diatoms — the  shores  and  bottom  of  the  river  and  its  tributaries. 
They  are  represented  in  the  plankton  at  all  seasons,  and  the  diver- 
gence in  numbers  is  at  no  time  so  marked  as  it  is  in  typical  plank- 
ton diatoms,  such  as  Asterionella. 

Nitzschia  amphioxys  (Ehrbg.)  Kutz.  appeared  several  times  in 
winter  collections,  and  N.  sigmoidea  (Nitzsch)  W.  Sm.  is  adventi- 
tious in  small  numbers  in  flood  waters.  Several  species  of  Pleu- 
rosigma  appear  at  irregular  intervals  throughout  the  year  in  both 
flood  waters  and  stable  conditions  and  are  apparently  adventitious, 
appearing  in  relatively  small  numbers. 

Rhizosolenia  eriensis  H.  L.  Smith  was  noted  on  a  few  occasions 
in  winter  planktons.  Its  exceeding  transparency  and  the  abun- 
dance of  silt  and  debris  at  the  times  of  its  occurrence  so  obscure  it 
that  it  may  have  escaped  detection  in  many  instances. 


58 

Stephanodiscus  niagarce  Ehrbg.,  a  common  planktont  in  the 
waters  of  the  Great  Lakes,  appeared  but  once,  in  May,  in  our  plank- 
ton, though  the  river  had  for  years  received,  by  way  of  the  Chicago 
River,  constant  access  of  water  from  Lake  Michigan.  The  turbid, 
sewage-laden,  and  warmer  waters  of  the  Illinois  are  evidently  not 
favorable  for  its  growth. 

Surirella  ovalis  Kiitz.  var.  minuta  (Breb.)  Kirchner.* — Average 
number,  761,538.  Present  sparingly  throughout  the  year,  but 
principally  during  summer  months.  Vernal  pulse  in  May. 

Surirella  spiralis  Kiitz. — Average  number,  1,612.  Less  abundant 
in  the  more  stable  conditions  of  1897.  This  species  is  most  abun- 
dant in  Quiver  Creek  and  Spoon  River.  Its  fluctuations  are  slight, 
irregular,  and  often  appear  with  flood  waters,  all  of  which  phenom- 
ena indicate  its  adventitious  character  in  the  river  plankton. 

Synedra  acus  Kiitz.* — Average  number,  36,558,462  (silk,  308,- 
330).  This  species  is  a  perennial  planktont,  appearing,  for  example, 
in  1898  in  every  collection.  It  has  a  highly  developed  and  shifting 
vernal  pulse,  and  an  inconstant  and  but  slightly  developed  autumnal 
or  hiemal  pulse.  The  vernal  pulse  appears  in  1895  on  April  9  at 
209,880;  in  1896  on  April  24  at  366,828;  in  1897  on  May  25  at 
2,620,800  (82,800,000*);  and  in  1898  on  May  10  at  9,043,200 
(813,600,000*).  The  second  pulse  appears  in  1895  on  November 
14  at  99,360;  in  1896  on  December  3  at  44,464;  in  1897  no  pulse 
occurs;  in  1898  it  occurs  on  November  8  at  19,000.  As  in  some 
other  diatoms,  there  are  minor  pulses  throughout  the  year,  though 
in  this  case  they  are  all  feebly  developed,  exceeding  100,000  (silk) 
in  but  a  single  instance.  The  minor  pulses  of  midwinter  often 
exceed  in  prominence  those  of  midsummer.  The  meteoric  char- 
acter of  the  vernal  pulse  is  very  pronounced  in  this  species  both  in 
the  suddenness  of  its  appearance  and  its  disappearance  and  in  the 
height  which  it  attains. 

The  variety  delicatissima  W.  Sm.  is  included  here  with  the  type 
acus.  During  the  autumn  of  1898  a  separate  record  was  kept  of 
the  two,  with  the  result  that  the  variety  appears  to  include  about 
four  fifths  of  the  individuals  at  that  season.  The  twTo  are  not 
readily  separated.  The  colorless  form  recently  described  by  Pro- 
wazek  ('00)  as  5.  hyalina  is  also  included,  and  it  is  not  uncommon 
when  S.  acus  is  abundant.  Colorless  forms  of  other  diatoms  of  the 
plankton,  as  Asterionella,  Melosira,  and  Fragilaria,  also  occur,  but 


59 

it  would  seem  from  the  intergradation  with  the  normal  condition 
that  it  is  a  phenomenon  of  physiological  import  rather  than  of 
specific  significance.  It  would  seem  desirable  that  experimental 
breeding  of  diatoms  should  be  employed  as  a  test  before  specific 
diagnoses  utilize  this  character. 

Synedra  capitata  Ehrbg.  is  occasionally  adventitious  in  the  plank- 
ton in  spring  months. 

Synedra  ulna  (Nitzsch)  Ehrbg.* — Average  number,  302,308 
(silk,  34,510).  This  appears  somewhat  irregularly  in  the  plankton, 
with  a  vernal  pulse  on  May  17  of  5,400,000  and  an  autumnal  one 
November  15  of  1,800,000.  It  is  abundant  on  the  ooze  of  exposed 
springy  shores  after  rapid  decline  of  the  river,  and  is  probably 
adventitious  in  the  plankton  to  some  extent  from  this  region. 

Tabellaria  fene  strata  Kiitz.,  which  is  exceedingly  abundant  in  the 
plankton  of  European  lakes  and  in  our  own  Great  Lakes,  was  found 
but  a  single  time  in  the  wraters  of  the  Illinois.  It  can  hardly  be  lack 
of  food  elements  which  prevents  its  development,  and  there  are 
times  when  favorable  thermal  conditions  would  seem  to  be  offered 
in  spring  and  autumn,  when  the  river  temperatures  do  not  exceed 
the  summer  temperatures  of  our  Great  Lakes.  It  may  be  that  the 
chemical  conditions  attending  sewage  contamination  exert  a  dele- 
terious influence  upon  this  species  and  others  of  the  genus,  such  as 
T.  flocculosa,  which  abound  in  purer  lake  waters. 

CONJUGATE. 

This  group  of  algae  is  represented  in  the  plankton  only  by  a  few 
desmids,  which  neither  in  number,  or  quantity,  play  any  important 
part  in  the  ecology  of  the  plankton.  The  filamentous  algae  are 
abundantly  represented  in  spring  in  the  backwaters  of  the  Illinois 
River,  where  they  form  extensive  littoral  fringes  of  "blanket  moss," 
which  load  down  the  emerging  littoral  flora.  This  fringe  is  fre- 
quently stranded  by  the  retreat  of  flood  waters.  In  some  localities, 
as  in  Phelps  Lake,  it  plays  a  very  important  part  in  the  food  cycle, 
since  by  its  decay,  as  temperatures  approach  the  summer  maximum, 
it  contributes  immediately  its  store  of  organic  nitrogen  to  the  sup- 
port of  the  small  algre  and  flagellates  which  develop  in  great  num- 
bers on  those  waters  at  that  season.  Some  species  of  Spirogyra 
and  Zygnema  have  a  habit  of  breaking  up  into  short  filaments,  and 


60 

in  this  condition  they  have  often  been  taken  in  some  quantity  in 
the  plankton  of  the  river,  but  they  are  so  plainly  adventitious  and 
irregular  that  no  notice  has  been  taken  of  them  in  our  enumeration 
work,  and  when  possible  they  have  been  removed  before  measure- 
ment or  deducted  by  estimation  from  the  volumetric  records. 

The  desmids  are  few  both  in  species  and  individuals.  Seven 
species  have  been  recognized,  of  which  but  four  are  of  general 
occurrence  in  the  plankton.  These  are  three  species  of  Closterium 
and  Staurastrum  gracile.  The  latter  and  Cosmodadium  saoconicum 
are  the  only  eulimnetic  organisms  among  them.  The  center  of  dis- 
tribution of  the  other  species  is  the  shore  and  bottom.  The  stom- 
achs of  fish  such  as  the  CatostomidcB,  the  carp,  and  Dorosoma  cepe- 
dianum,  which  often  feed  upon  the  bottom  ooze  or  slime  about 
aquatic  plants,  usually  contain  many  desmids,  including  the  species 
here  noted.  Other  species  also  are  occasionally  adventitious  in  the 
plankton,  and  the  list  might  be  considerably  extended,  though  the 
absence  of  extensive  peat  bogs  in  the  drainage  basin  of  the  river 
reduces  the  desmids  to  a  position  of  much  less  importance  than  that 
which  they  occupy  in  more  northerly  waters. 

As  a  group  they  exhibit  a  well-defined  seasonal  distribution, 
with  a  vernal  pulse  at  about  the  time  of  the  volumetric  maximum 
in  April-May  and  an  autumnal  pulse  of  less  regular  occurrence, 
location,  and  size.  The  optimum  temperature  for  their  appear- 
ance in  the  plankton  lies  below  70°,  and  in  winter  months  they 
occur  but  rarely. 

DISCUSSION    OF    SPECIES    OF    CONJUGATE. 

Closterium  acerosum  Ehrbg. — Average  number,  348.  More  than 
three  times  as  abundant  in  the  previous  year.  This  desmid  is 
perennial  in  the  plankton,  having  been  found  in  every  month  of  the 
year,  but  at  irregular  intervals,  and  never  in  large  numbers.  Its 
distribution  is  such  as  to  suggest  that  it  is  at  the  most  only  semi- 
limnetic  in  habit.  The  numbers  are  too  small  to  follow  closely  the 
seasonal  distribution.  There  are  pulses  on  May  3  (3,200),  Septem- 
ber 6  (2,400),  and  November  1  (2,500)  in  1898;  and  in  1897  a  pulse 
on  June  28  (2,000)  and  one  on  September  21  (24,000).  In  previ- 
ous years  vernal  pulses  in  April  and  occasional  autumnal  pulses  are 
to  be  noted.  In  so  far  as  the  optimum  temperature  is  indicated,  it 


61 

seems  not  to  lie  near  either  extreme,  and  above  rather  than  below 
the  average  for  the  year. 

Closterium  gracile  Breb.* — Average  number,  49,616  (silk,  305). 
This  species  was  found  in  small  numbers  from  March  to  December, 
and  shows  pulses  on  May  17  (1,600)  and  September  ~27"(6,400)  at 
temperatures  of  64°  and  73°.  The  tenuity  of  the  form  of  the 
frustule  of  this  species  suggests  a  limnetic  habit. 

Closterium  hmula  Ehrbg. — Average  number,  556.  This  also  is 
a  perennial  species,  and  is  somewhat  more  abundant  and  constant 
than  C.  acerosum.  It  likewise  has  a  vernal  pulse,  which  in  1895 
appears  on  April  29  (2,915) ;  in  1896,  on  May  1  (5,364) ;  in  1897,  on 
May  25  (3,200);  and  in  1898,  on  May  24  (6,000).  In  both  this 
species  and  C.  acerosum  there  are  slight  indications  of  recurrent 
minor  pulses  which  are  often  coincident  in  the  two  species.  Nine 
such  movements  appear  in  1898.  The  autumnal  pulses  are  less 
regular  in  their  appearance  and  size  than  the  vernal,  and  appear 
from  September  to  November.  The  optimum  temperatures  seem 
to  lie  between  45°  and  70°.  This  species  is  only  semi-limnetic,  and 
never  attains  the  fluctuations  which  characterize  most  limnetic 
organisms.  Doubtless  other  so-called  species  of  Closterium  have 
been  included  among  the  variable  organisms  referred  here  to  C. 
lumila  and  C.  acerosum. 

Cosmarium  constrictum  Delp.  was  found  occasionally  from  March 
to  September,  and  is  probably  adventitious. 

Cosmocladium  saxonicum  De  By. — A  single  isolated  pulse  of  this 
minute  limnetic  desmid  appeared  in  the  filter  collections  of  Septem- 
ber, 1897.  It  was  first  noted  on  August  31  and  disappeared  after 
September  29,  and  was  never  found  at  other  times  in  the  plankton. 
The  pulse  culminated  September  9  at  13,500,000*. 

Gonatozygon  brebissonii  De  By. — The  filaments  of  this  desmid 
were  noted  in  the  plankton  only  in  March,  1899,  attaining  a  maxi- 
mum of  136,800  on  the  14th. 

Staurastrum  gracile  Ralfs. — Average  number,  31.  About  two 
hundred  times  as  abundant  in  the  plankton  of  1897.  It  occurs  from 
March  to  January.  No  vernal  pulse  was  detected,  but  an  autumnal 
one  of  14,000  appears  September  29.  It  appears  in  much  larger 
numbers  in  the  filter-paper  collections,  and  is  probably  a  limnetic 
planktont  in  our  waters. 


62 

Undetermined  species  of  Penium,  Arthrodesmus,  and  Docidium 
have  been  found  in  the  plankton  but  always  singly.  They  are 
doubtless  adventitious. 

PHANEROGAMIA. 

The  Lemnacecz  are  represented  in  our  waters  by  several  species 
of  Lemna,  by  Spirodela,  and  by  two  species  of  Wolff ia — brasiliensis 
and  columbiana.  The  first  two  genera  are  predominantly  floating 
surface-plants,  while  the  last  occurs  at  all  levels,  is  taken  with  the 
plankton,  and  has  been  treated  in  our  measurements  and  enu- 
merations as  a  limnetic  organism. 

Wolff  ia  brasiliensis  Weddell. — Average  number,  2 ;  in  1897,  13. 
It  appears  irregularly  in  river  planktons  from  the  last  of  March  till 
January,  and  is  somewhat  more  abundant  in  late  summer  and 
autumn.  The  seining  operations  of  fishermen  in  the  river  and 
tributary  backwaters  have  much  to  do  with  its  appearance  in  the 
plankton  of  the  river. 

Wolff  ia  columbiana  Karsten. — Average  number,  7;  in  1897,  41. 
With  the  preceding  species.  Neither  of  these  species  are  sufficiently 
abundant  greatly  to  affect  the  ecology  or  quantity  of  the  plankton 
of  the  river,  though  they  are  of  more  importance  in  the  backwaters. 
Owing  to  their  size  and  duration  they  compete  with  the  smaller 
organisms  of  the  phytoplankton,  but  do  not  serve  as  food  for  any  of 
the  zooplankton. 

PROTOZOA. 

Average  number,  111,731,000.  The  number  of  species  exceeds 
147  (  +  38),  distributed  as  follows:  Mastigophora,  60 (  +  10);  Rhizop- 
oda,3l  (  +  28);  Heliozoa,5;  Sporozoa  (3) ;  Ciliata,  45;  and  Suctoria, 
5, — the  numbers  in  parentheses  indicating  the  additional  forms 
whose  specific  rank  was  not  recognized  in  the  enumerations . 

The  Protozoa  occur  in  great  numbers  (Table  I.)  in  every  collection 
of  the  year.  Owing  to  the  fact  that  the  totals  are  a  conglomerate  of 
two  methods  of  collecting,  of  a  large  number  of  species  of  many  di- 
vergent seasonal  tendencies,  and  of  both  eulimnetic  and  adventitious 
forms,  their  seasonal  fluctuations  have  no  particular  significance  which 
is  not  better  treated  either  in  connection  with  the  subdivisions  of  the 
class  or  with  the  individual  species.  In  the  totals,  traces  appear  of 


63 

the  vernal  pulse,  of  the  midsummer  maximum  of  the  chlorophyll- 
bearing  Mastigophora,  and  of  the  autumnal- winter  wave  of  Ciliata. 
The  Protozoa,  through  the  Mastigophora,  share  with  the  algas 
the  synthetic  function  in  the  elaboration  of  food  from  inorganic  or 
partially  disorganized  organic  contents  of  the  water.  _They  utilize 
decaying  organic  matter  as  food,  and  are  thus  primary  links  in  the 
cycle  of  food  relations.  Some  of  them  feed  upon  bacteria,  upon  alga3, 
or  even  upon  other  animals,  and  thus  become  secondary  or  tertiary 
links  in  the  chain. 

MASTIGOPHORA. 
(Plates  I.  and  II.) 

Average  number,  including,  without  duplication,  both  silk  and 
filter-paper  collections,  95,856,449.  In  the  collections  of  1897 
they  were  five  times  as  abundant  as  a  result,  in  part  at  least,  of  the 
extended  low-water  period,  sewage  contamination,  and  extension 
of  high  temperatures  during  the  late  autumn  of  that  year  (Pt.  I., 
PI.  XL). 

The  Mastigophora  abound  in  every  collection  and  occur  at  all 
seasons  of  the  year.  Four  fifths  of  them  occur,  however,  between 
the  first  of  April  and  the  last  of  September.  They  are  predominant- 
ly chlorophyll-bearing  organisms,  and  have  their  greatest  numbers 
during  the  same  season  in  which  the  land  flora  attains  its  growth. 
They  spring  into  abundance  with  the  opening  buds  of  April,  and  van- 
ish from  the  plankton  when  frost  cuts  off  the  foliage  in  autumn. 
There  are,  it  is  true,  some  species,  such  as  Synura,  which  grow 
luxuriantly  at  winter  temperatures,  but  these  are  generally  of  the 
chrysomonad  type,  with  yellowish  or  brownish  chromoplasts.  The 
bright  green  chlorophyll-bearing  flagellates  are  in  the  main  summer 
planktonts.  Since  water  temperatures  do  not  fall  below  32°,  the 
phytoplankton  is  exempt  from  this  risk  of  destruction  against  which 
the  land  flora  must  provide.  We  find,  accordingly,  that  the  most  of  the 
Mastigophora  are  wont  to  occur  in  diminished  numbers  and  irregu- 
larly in  the  plankton  throughout  the  winter.  This  appears  in  the 
records  of  the  more  common  species,  and  fuller  examination  would 
doubtless  greatly  increase  the  number  which  thus  winter  over  in 
reduced  numbers. 

I  have  already  called  attention  to  the  fact  that  there  are  in 
1898-99  recurrent  pulses  in  the  Chl0rophyce&  and  Bacillariacece  at 


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66 

intervals  of  several  weeks,  and  that  such  pulses  can  also  be  traced 
back  into  1897  as  far  as  the  collections  were  made  at  weekly  inter- 
vals— that  is  to  the  early  part  of  July.  A  similar  periodicity  on  the 
part  of  the  Mastigophora — the  greater  part  of  which  are  also 
chlorophyll-bearing — is  even  more  evident.  Not  •  only  is  this 
periodicity  present  in  this  group,  but  it  coincides  approximately  in 
the  location  of  its  maxima  and  in  their  relative  development  with 
that  found  in  the  Chlorophycece  and  BacillariacecB.  The  following 
table,  which  gives  the  dates  of  culmination  of  the  pulses  of  these  three 
groups  from  July  1,  1897,  to  April  1,  1899,  will  serve  to  demonstrate 
this  point  more  clearly,  and  a  graphic  presentation  of  the  data  will 
be  found  in  Plates  I.  and  II. 

There  are  twenty-two  of  these  recurrent  pulses  in  the  period 
from  July,  1897,  to  March,  1899.  Of  the  sixty-six  possible  maxima 
only  five  are  missing,  or  at  least  not  apparent  in  our  data,  and  but 
ten  culminate  on  other  dates  than  the  one  (of  collection)  most  to  be 
expected.  These  ten  in  every  case  culminate  either  a  week  prior  or 
subsequent  to  that  in  which  the  other  two  groups  reach  their  max- 
ima. These  divergences  may  be  due  to  the  error  incident  to  the 
interval  of  collection,  and  their  approximation  in  time  is  still  cor- 
roborative of  the  tendency  towards  recurrent  periods  of  growth. 
These  exceptions  are  no  greater  than  might  be  expected  to  occur  in 
the  unstable  fluviatile  environment  and  within  the  large  margin  of 
error  of  the  plankton  method. 

There  are  twenty-one  intervals  between  July  14,  1897,  and 
March  14,  1899,  with  a  range  in  length  of  20  to  42  days  and  an  aver- 
age of  28.95.  The  intervals  in  days  with  the  numbers  of  instances 
of  each  are  as  follows:  20  (1),  21  (3),  22  (1),  23  (1),  26  (1), 
27  (1),  28  (7),  35  (3),  and  42  (3),  days.  The  effect  of  the  weekly 
interval  of  collection  is  seen  in  the  preponderances  at  21,  28,  35,  and 
perhaps  at  42,  days.  There  is  evidently  a  tendency  towards  the 
interval  of  28  days.  Nine  of  the  21  pulses  are  grouped  about  this 
interval;  6,  about  that  of  21 ;  while  3  are  at  35  and  3  at  42.  If  there 
be  such  a  tendency  it  is  but  natural  that  with  a  weekly  interval  of 
collection  there  should  also  appear  minor  preponderances  at  2 1  and 
35  days.  Traces  of  a  similar  rhythm  may  be  found  in  the  period  of 
weekly  collections  in  1896  (Pt.  I.,  Table  III.). 

In  some  instances  the  environmental  conditions  at  these  times 
of  departure  are  such  as  to  suggest  that  they  may  have  produced  the 


67 

shifting  in  the  position  of  the  maxima.  Thus  the  pulse  of  January 
25,  1898,  appears  after  a  3 5 -day  interval,  but  in  the  midst  of  the 
rising  winter  flood,  to  whose  effect  the  delay  may  be  attributed. 
In  both  1896  and  1898  the  28-day  rhythm  is  interrupted  at  the  time 
of  the  vernal  pulse  in  April-May.  It  appears  as  though  these  re- 
current pulses — -if  such  exist — were  submerged  in  the  greater  ver- 
nal increase.  The  double  summit  of  the  vernal  pulse  in  the  curve 
of  the  Bacillariacea  and  Mastigophora  (PI.  II.)  for  1898  suggests  the 
compound  character  of  this  pulse  in  the  case  of  these  groups  of 
organisms  at  least.  The  time  interval  in  the  case  of  the  vernal  in- 
terruption is  also  significant.  In  1898  there  are  two  pulses  between 
March  22  and  July  19,  at  intervals  of  42  days — a  total  of  84  days, 
which  is  the  equivalent  in  duration  of  three  28-day  intervals. 

The  total  number  of  species  of  Mastigophora  recorded  by  me 
from  the  plankton  of  the  Illinois  River  is  over  sixty.  This  number 
will  be  increased  to  more  than  seventy  if  forms  not  separated  in  our 
enumerations  be  distinguished  as  separate  species. 

The  Protomastigina  (including  the  Bicoscecidce  and  the  Cras- 
pedomonadida]  are  well  represented  in  the  plankton  by  passive 
limnetic  species  which  are  principally  sessile  on  other  planktonts. 
These  are  Bicosoeca  lacustris,  Salpingceca  brunnea,  S.  minuta,  and 
Diplosiga  frequentissima.  Asterosiga  radiata  is  a  eulimnetic  repre- 
sentative and  Anthophysa  vegetans  an  adventitious  one.  As  a  group 
they  are  more  abundant  during  the  warmer  part  of  the  year. 

The  Chrysomonadidcz  are  also  well  represented,  and  include  the 
most  abundant  flagellates  of  the  plankton  of  the  colder  months. 
Synura  uvella  is  quantitatively  the  largest  factor  furnished  by  this 
group.  It  is  supplemented  by  Syncrypta  volvox,  and  the  various 
forms  of  Dinobryon,  Uroglena,  and  Mallomonas.  The  last  two 
genera  have  more  of  a  summer  range  of  occurrence,  but  are  not  of 
quantitative  importance  in  the  waters  of  the  Illinois. 

The  CryptomonadidcB  are  represented  only  by  Chilomonas  and 
Cryptomonas,  and  are  of  somewhat  constant,  though  of  minor, 
importance  quantitatively. 

The  Euglenidce,  on  the  other  hand,  are,  in  our  waters  at  least, 
second  to  no  coordinate  group  in  their  quantitative  importance. 
They  are  individually  of  relatively  large  size,  and  they  occur  in 
great  numbers  throughout  the  summer  months,  replacing  the 
Chrysomonadida  of  the  colder  seasons  of  the  year.  Euglena 

(6) 


68 

viridis  is  the  most  abundant,  and  it  is  associated  with  other  species 
of  the  genus,  with  species  of  Amblyophis,  Phacus,  Lepocindis,  Chlo- 
ropeltis,  Colacium,  and  Trachelomonas ,  especially  the  latter. 

The  PeridimidcB  are  quantitatively  of  considerable  importance 
in  the  plankton  of  our  Great  Lakes  (Kofoid,  '95),  but  in  the  Illinois 
River  they  are  of  little  significance,  at  least  the  larger  forms  such  as 
Ceratium.  Smaller  species  such  as  Peridinium  tabulatum  and 
Glenodinium  cinctwn  are  more  abundant.  As  a  group  they  do  not 
show  any  marked  seasonal  preferences. 

The  Volvocida,  on  the  other  hand,  are  of  more  than  the  usual 
consequence  in  the  plankton  of  the  Illinois.  The  group  is  repre- 
sented by  the  curious  Chloraster  gyrans,  by  the  sporadic  and  meteor- 
ic Carteria  multifttis,  and  by  the  colonial  genera  Eudorina,  Pando- 
rina,  Pleodorina,  Platydorina,  and  Volvox.  As  a  group  they  are 
almost  exclusively  summer  planktonts. 

The  Mastigophora  as  a  whole  are,  next  to  the  BacillariacecB,  the 
most  abundant  of  the  synthetic  organisms  of  the  plankton.  Their 
quantitative  importance  has  not  hitherto  been  sufficiently  demon- 
strated in  the  plankton  of  fresh  water,  owing  it  may  be  to  their 
escape  through  the  silk  net  in  the  ordinary  methods  of  collection. 
It  seems  quite  probable  also  that  they  may  be  present  in  our  warm 
and  fertile  waters  in  much  greater  abundance  than  they  are  in  the 
colder  and  clearer  waters  of  most  lakes.  This  is  especially  true  of 
the  EuglenidcB  and  Volvocidcz,  perhaps  less  so  of  the  ChrysomonadidcB 
and  Peridiniidce. 

DISCUSSION    OF    SPECIES    OF    MASTIGOPHORA. 

Amblyophis  viridis  Ehrbg.* — Average  number,  63,014  in  1897. 
It  occurred  throughout  the  summer  in  1897,  from  May  to  October, 
with  a  maximum  of  1,440,000  on  August  31.  Apparently  a  sum- 
mer planktont  but  never  very  abundant. 

Anthophysa  vegetans  (O.  F.  Mull.)  Butschli. — This  was  identi- 
fied in  the  plankton  of  June,  1898.  It  is  very  abundant  at  times  on 
various  substrata  in  stagnating  water,  and  from  such  places  becomes 
adventitious  in  detached  fragments  of  colonies  in  the  plankton. 

Aster osiga  radiata  Zach. — This  interesting  colonial  and  limnetic 
choanoflagellate,  described  originally  from  the  plankton  of  German 
lakes,  has  been  found  but  a  single  time  in  our  plankton — in  the  latter 


69 

part  of  August,  1896'.  It  is  one  of  many  illustrations  of  the  cosmo- 
politan distribution  of  plankton  organisms. 

Bicosceca  lacustris  J.  Clark*. — Average  number,  112,896.  Only  one 
third  as  abundant  in  1896,  and  four  times  as  many  in  1897.  This 
minute  flagellate  is  found  in  our  waters  sessile  upon  the-filaments  of 
Melosira,  principally  M.  granulata  var.  spinosa.  It  occurs  more 
frequently  upon  the  dead  frustules  than  upon  live  ones,  and  upon 
those  of  the  shorter  form  than  upon  the  longer.  It  has  appeared 
also  upon  Dinobryon  sertularia,  Pediastrum  pertusum,  and  Richteri- 
ella  botryoides.  It  exhibits  a  considerable  range  of  variation  in 
proportions,  in  the  amount  of  lateral  compression,  and  in  the  length 
of  the  pedicels.  These  variable  forms  are,  however,  connected 
with  the  type  as  described  by  Clark,  and  are  not,  in  my  opinion,  to 
be  designated  as  distinct  species.  Zacharias  ('94)  has  described 
one  of  these  variants  as  B.  oculata.  I  regard  it  as  a  growth  condi- 
tion of  B.  lacustris,  and  not  as  specifically  distinct  from  it. 

Its  seasonal  distribution  in  1898  is  somewhat  peculiar.  It 
appears  as  two  quite  symmetrical  pulses,  the  first  extending  from 
early  in  June  till  the  middle  of  July,  and  culminating  on  June  14  at 
3,801,600.  The  approach  of  this  pulse  is  abrupt  and  its  decline 
somewhat  gradual.  The  species  does  not  reappear  until  September 
13.  The  autumnal  pulse  culminates  October  11  at  486,000,  then 
gradually  declines,  and  disappears  November  1.  There  is  no  record 
of  its  occurrence  in  1898  outside  of  these  two  pulses.  In  1897  it  is 
found  irregularly  from  May  to  August,  and  in  1896  in  February  and 
from  May  to  December,  with  pulses  in  May,  June,  July  (2) ,  August, 
and  October. 

In  1898  its  optimum  temperatures  appear  at  82°  and  65°,  and  its 
pulses  in  other  years  do  not  occur  below  57°.  It  thus  belongs  to  the 
plankton  of  the  warmer  months. 

Its  seasonal  distribution  falls  within  that  of  the  limits  of  its  host 
Melosira,  and  in  1896  and  1898  their  vernal  pulses  coincide,  and  the 
same  correlation  appears  in  all  but  one  of  the  pulses  of  1896.  Not 
all  Melosira  pulses,  however,  are  attended  by  an  increase  in  Bico- 
sceca. Thus  in  the  late  summer  and  fall  of  1897  Melosira  fluctuated 
without  any  appearance  of  Bicosceca.  In  the  autumn  of  1898  the 
pulse  of  Bicosceca  on  October  1 1  appears  on  the  decline  of  the  Sep- 
tember pulse  of  Melosira,  in  which  the  host  made  no  corresponding 
increase.  Melosira  is  thus  apparently  essential  for  any  marked  in- 


70 

crease  of  Bicosceca  in  the  plankton,  but  is  not  in  itself  the  primary 
cause  for  its  appearance  in  the  plankton. 

Carteria  multifilis  (Fres.)  Dill.* — Average  number,  2,365,384. 
In  1897  more  than  one  hundred -fold  as  abundant.  This  species  was 
recognized  only  in  the  autumnal  and  hiemal  planktons,  from 
August  till  January  in  1897-98  and  from  October  to  February  in 
1898-99.  It  is  not  easily  and  with  certainty  identified  by  the 
usual  methods  of  plankton  counting,  and  probably  other  species  of 
similar  habitus  may  have  been  included  to  some  extent ;  and,  on  the 
other  hand,  many  Carteria  may  have  been  thrown  with  the  "un- 
identified" flagellates,  especially  in  earlier  years.  This  species 
occurs  throughout  the  whole  range  of  temperatures,  and  its  maxi- 
mum development  (6,476,400,000)  was  attained  October  5,  1897,  at 
70°.  A  pulse  prior  to  this  appeared  September  7,  at  2,846,250,000. 
From  the  major  pulse  in  October  there  is  a  gradual  decline  as  the 
minimum  temperatures  are  reached. 

The  remarkable  outbreak  of  Carteria  in  the  autumn  of  1897  was 
associated  with  unusually  low  water  (Pt.  I.,  PI.  XI.)  and  concen- 
tration of  sewage  and  decrease  in  current.  The  water  of  the  stream 
was  of  a  livid  greenish-yellow  tinge,  due  principally  to  great  numbers 
of  Carteria,  which  developed  to  the  exclusion  or  diminution  of  other 
chlorophyll-bearing  flagellates  such  as  Euglena,  and  of  diatoms  such 
as  Melosira.  This  unusual  development  seems  to  have  been  a  dis- 
turbing factor  in  the  usual  seasonal  routine  of  the  autumnal  plank- 
ton of  that  year. 

The  distribution  of  Carteria  in  the  river  was  remarkable.  It 
formed  great  bands  or  streaks  visible  near  the  surface,  or  masses 
which  in  form  simulated  cloud  effects.  The  distribution  was 
plainly  uneven,  giving  a  banded  or  mottled  appearance  to  the 
stream.  The  bands,  10  to  50  meters  in  width,  ran  with  the  channel 
or  current,  and  their  position  and  form  were  plainly  influenced  by 
these  factors.  No  cause  was  apparent  for  the  mottled  regions. 
This  phenomenon  stands  in  somewhat  sharp  contrast  to  the  distri- 
bution of  the  usual  water-bloom  upon  the  river,  which  is  generally 
composed  largely  of  Euglena.  This  presents  a  much  more  uniform 
distribution,  and  unlike  the  Carteria  is  plainly  visible  only  when  it 
is  accumulated  as  a  superficial  scum  or  film.  Carteria  was  present 
in  such  quantity  that  its  distribution  was  evident  at  lower  levels 
so  far  as  the  turbidity  would  permit  it  to  be  seen.  It  afforded  a 


71 

striking  instance  of  marked  inequalities  in  distribution  within 
small  areas,  of  at  least  one  plankton  organism. 

Carteria  showed  great  variation  in  the  amount  of  chlorophyll 
present.  Some  individuals  were  practically  colorless.  It  seems 
very  probable  that  in  the  presence  of  great  abundance^  partially 
decayed  organic  matter  such  as  occurs  in  a  sewage-laden  stream, 
Carteria  may  become  largely  holozoic  in  its  nutrition,  as  Zumstein 
('99)  has  shown  to  be  the  case  with  Euglena.  The  literature  of 
fresh-water  plankton  contains  no  record  of  a  similar  preponderance 
of  Carteria  in  other  localities,  though  its  occurrence  has  been  occa- 
sionally noted  in  the  plankton. 

The  chemical  conditions  under  which  this  great  pulse  of  Carteria 
appeared  in  the  autumn  of  1897  can  be  followed  in  Part  I.,  Plate 
XL  IV  and  Table  X.  The  high  chlorine  and  the  great  increase  in  free 
ammonia  and  nitrites  indicate  the  decay  of  sewage;  the  high 
nitrates  and  albuminoid  ammonia  show  that  there  was  no  lack  of 
some  at  least  of  the  important  sources  of  food.  The  two  principal 
pulses  appear  September  7  (2,846,250,000)  and  October  5  (6,476,- 
400,000),  with  a  minimum  of  680,400,000,  on  September  21,  sep- 
arating them.  Both  of  these  pulses  are  attended  by  sharp  declines 
in  nitrates  and  nitrites  and  free  ammonia,  and  very  slight  decreases 
in  organic  nitrogen  and  albuminoid  ammonia.  Either  the  first 
three  substances  named  or  those  matters  which  supply  them  by 
their  decay,  are  thus  noticeably  utilized  at  the  times  of  these  pulses. 

The  relation  of  the  Carteria  to  the  volumetric  pulses  is  (Pt.  I., 
PI.  XI.)  not  a  constant  one.  The  Carteria  pulse  of  September  7 
lies  in  a  slight  depression  between  two  maxima  of  the  volumetric 
curve,  and  a  week  prior  to  the  autumnal  culmination  on  September 
14  at  19.8  cm.3  per  m.3.  It  thus  appears  during  the  growth  period 
of  this  volumetric  maximum.  The  second  and  larger  pulse  of 
Carteria,  on  October  5,  coincides  with  the  second  volumetric  maxi- 
mum, and  in  fact  fluctuates  throughout  with  it.  Though  Carteria 
constitutes  but  a  small  part  of  the  actual  catch  of  the  silk  net, 
owing  to  leakage  through  the  silk,  it  is  apparently  an  important 
factor  in  the  food  cycle  which  builds  up  such  maxima. 

Ceratium  brevicorne  Hempel. — This  species  appeared  in  small 
numbers  in  isolated  instances  from  April  through  October.  It 
varies  towards  C.  hirundinella,  but  the  small  numbers  in  which  it 
has  occurred  have  not  as  yet  afforded  sufficient  ground  for  regard- 


72 

ing  it  as  a  variety  of  that  species.  It  occurs  most  frequently  in 
August  and  September,  and  is  apparently  a  warm-water  planktont. 

Ceratium  cornutwn  Ehrbg.  was  found  but  once — in  June,  1896. 

Ceratium  hirundinella  O.  F.  Mull,  was  not  noted  in  our  plank- 
ton in  1898,  but  in  1896  was  found  from  June  to  October,  with  a 
pulse  of  19,200  on  June  6.  It  was  recorded  only  at  temperatures 
above  57°,  and  is  apparently  a  warm-water  planktont.  It  has  but 
an  insignificant  part  in  the  potamoplankton  of  the  Illinois  River 
and  its  backwaters,  though  quite  abundant  in  the  summer  plank- 
ton of  Lake  Michigan  (Kofoid,  '95).  It  seems  not  to  have  survived 
the  transit  through  the  sewage-laden  waters  of  Chicago  River  or  to 
thrive  in  the  conditions  prevailing  in  the  Illinois  River,  though 
common  generally  in  fresh- water  plankton  of  the  temperate  zone. 

Chilomonas  paramcecium  Ehrbg.* — Average  number,  555,000. 
This  flagellate,  which  is  frequently  abundant  in  aquaria  or  stag- 
nant water,  appears  also  in  the  plankton  of  the  Illinois  River. 
There  is  in  1898  a  vernal  pulse,  culminating  at  10,800,000  on  April 
26,  and  there  are  scattered  records  from  October  to  February. 

Chloraster  gyrans  Ehrbg. — This  rare  and  unique  flagellate  was 
found  in  but  two  collections — in  July  and  August,  1898 — and  only 
in  small  numbers. 

Chloropeltis  monilata  Stokes.* — Average  number,  362,941  in 
1897.  This  is  a  summer  planktont,  appearing  at  irregular  inter- 
vals from  the  last  of  May  until  the  middle  of  September.  It  was 
not  found  in  1898.  A  maximum  of  10,800,000  appears  on  August 
31. 

Colacium  calvum  Stein. — The  attached  stage  only  of  this  flagel- 
late was  observed ,  and  was  recorded  only  in  1 8  9  6  and  1897.  It  appears 
from  the  middle  of  April  to  the  first  of  October,  and  is  usually 
found  upon  Polyarthra  platyptera.  It  has  occurred  occasionally 
upon  several  species  of  Brachionus  and  upon  Chydorus  sphczricus. 
The  largest  number  recorded  (162,792)  appeared  on  April  17,  1896, 
upon  Polyarthra,  usually  upon  the  body  and  more  rarely  upon  the 
oar-like  appendages.  It  is  often  exceedingly  abundant  upon  the 
planktonts  of  backwater  ponds. 

Colacium  vesiculosum  Ehrbg. — This  species  is  much  less  abun- 
dant than  the  preceding  species  in  our  waters,  and  was  found  only 
in  June  and  September,  upon  Cyclops  albidus  and  Polyarthra. 


73 

Cryptomonas  ovata  Ehrbg.* — Average  number,  121,154.  This 
species  has  been  recorded  principally  in  the  autumnal  or  hiemal 
plankton.  It  escapes  through  the  silk  net  readily,  and  was  rarely 
found  in  collections  of  earlier  years.  In  1895  it  occurred  from  July 
till  the  last  of  October,  and  in  1898  was  common  in  the  December 
plankton. 

Dinobryon  sertularia  Ehrbg. — Like  most  typical  planktonts, 
Dinobryon  is  an  exceedingly  variable  organism,  and  the  varia- 
tion finds  its  expression  in  the  form  and  proportions  of  the 
loricas  and  in  their  arrangement  and  continuity  in  colonies. 
Divergences  from  described  and  figured  species  are  thus  at  once 
apparent,  and  they  have  been  utilized  by  systematists,  notably  by 
Lemmermann  ('00)  and  by  Brunnthaler  ('01)  as  the  basis  for  the 
establishment  of  a  large  number  of  new  species.  The  validity  of 
these  species,  in  my  opinion,  must  rest  ultimately  upon  careful 
experimental  evidence  of  their  present  mutual  genetic  independence 
under  normal  conditions  of  growth.  From  my  own  observations 
upon  large  numbers  of  colonies  and  individuals  distributed  through- 
out the  range  of  their  seasonal  recurrence  in  six  years  in  our  waters, 
I  am  inclined  to  regard  all  as  belonging  to  a  single  species,  and  the 
different  types  as  mere  growth  varieties.  The  rapidity  of  growth 
and  the  age  of  the  individual  or  of  the  colony  are,  I  believe,  impor- 
tant factors  in  the  determination  of  the  form  of  the  lorica,  and  its 
various  forms  are  therefore  not  of  specific  value,  but  rather  of 
physiological  significance.  It  is  a  simple  matter  to  find  individuals, 
or  even  colonies,  conforming  to  the  descriptions  of  the  several 
species,  but  it  is  not  so  easy  to  refer  all  individuals  and  all  colonies 
to  the  described  types.  They  intergrade — nay,  more,  two,  or  even 
more,  "species"  are  not  infrequently  combined  in  the  same  colony. 
I  have  never  found  all  the  forms  in  a  single  colony,  but  such  com- 
binations as  angulatum-divergens,  diver gens-angulatum-stipitatum, 
sertularia-angulatum,  and  sertularia-undulatum  have  been  observed 
by  me.  These  combinations  are  most  frequent  in  large  colonies, 
and,  indeed,  the  number  of  "species"  in  a  colony  is  apparently  a 
function  of  its  size.  The  slender  growing  tips  are  wont  to  assume 
the  stipitatum  type  of  lorica  and  colony,  and  the  older  loricas  at  the 
base  to  conform  to  that  of  sertularia,  divergens,  or  angulatum. 
Small  colonies  as  a  rule  belong  to  a  single  "species."  These  com- 
binations are  generally  most  evident  during  the  maximum  period 


74 

of  growth;  that  is,  when  Dinobryon  is  multiplying  rapidly,  though 
they  may  appear  at  any  season  of  its  occurrence. 

In  the  enumeration  of  Dinobryon  five  types  were  recognized,  and 
the  individuals  were  assorted  to  these  "species,"  viz. :  D.  sertularia, 
stipitatum,  diver  gens,  angulatum,  and  undulatum.  Some  corrobora- 
tion  of  the  view  that  we  are  dealing  with  a  single  variable  organism 
and  not  with  five  distinct  species  may  be  seen  in  the  coincidence  of 
the  seasonal  distribution,  and  of  the  rise,  culmination,  and  decline 
of  the  pulses  of  the  five  different  forms. 

Since  these  varieties  have  such  a  similar  seasonal  distribution  I 
shall  treat  them  as  a  whole,  discussing  subsequently  any  individual 
peculiarities  which  are  noteworthy.  The  average  number  of 
individuals  of  Dinobryon  sertularia,  including  all  its  varieties,  in 
1898  was  1,979,785.  In  1897  the  average  was  much  smaller 
(79,352)  owing  to  the  few  collections  in  the  winter,  when  it  is  most 
abundant,  and  to  its  suppression  in  the  prolonged  low  water  of  the 
autumn  of  that  year.  The  relative  frequency  of  these  different 
varieties — for  I  shall  treat  them  as  such — is  shown  by  the  average 
per  cubic  meter  for  the  year  in  1898,  viz.:  D.  sertularia,  407,602; 
D.  sertularia  var.  stipitatum,  603,911;  D.  sertularia  var.  divergens, 
866,083;  D.  sertularia  var.  angulatum,  101,358;  D.  sertularia  var. 
undulatum,  831.  These  figures  are  only  approximate,  since  colonies 
containing  more  than  one  variety  have  all  been  included  with  the 
predominant  variety  in  the  colony,  which  is  usually  sertularia  or 
divergens,  consequently  angulatum  and  undulatum  are  more 
numerous  than  indicated  by  these  figures. 

The  seasonal  distribution  of  Dinobryon  in  our  waters  is  well 
defined,  and  is  sharply  limited  to  the  period  from  November  to  June. 
Its  earliest  recorded  appearance  was  November  8  in  1898,  while  in 
1896  and  1897  it  was  not  found  until  in  December.  It  lingers  well 
into  June  in  1896  and  1898 — the  two  years  in  which  the  spring 
collections  were  of  sufficient  frequency  to  trace  its  decline.  In  1898 
the  latest  record  was  on  June  28.  Most  of  the  records  after  May 
are  irregular  and  sporadic.  It  is  thus  absent  from  the  plankton  of 
the  Illinois  River  from  the  last  of  June  till  November  or  December. 
In  1895-1896  there  was  also  a  winter  interval  in  which  no  Dino- 
bryon was  recorded  during  the  December- January  flood  (Pt.  I.,  PI. 
IX.  and  X.).  In  1897-1898  a  similar  interval  appears,  and  con- 
tinues almost  to  the  end  of  the  slow  rise  of  the  flood  which  culmi- 


75 

nated  in  March.     Rising  floods  thus  do  not  favor  the  development 
of  Dinobryon  in  channel  waters  of  the  Illinois. 

The  interval  of  collection  in  1894-95  is  too  great  to  trace  the 
seasonal  fluctuations  of  Dinobryon,  though  there  are  indications  of 
a  maximum  pulse  on  April  29.  In  1895-96  there  is^  a  slight  de- 
velopment in  November  prior  to  the  rise  of  December,  in  which 
Dinobryon  again  disappears.  A  slight  pulse  of  3,192  appears  on  the 
declining  flood  (Pt.  I.,  PI.  X.)  on  January  25,  and  declines  again 
with  the  rise  in  February  to  reappear  on  February  20  at  42,588. 
Another  decline  in  Dinobryon  attends  the  rise  in  river  levels  in 
February-March,  and  after  a  fortnight  of  falling  levels  a  third 
pulse  of  2,531,280  is  seen  on  March  17.  Two  other  pulses  attend 
the  decline  of  this  flood,  one  upon  April  29  (800,064)  and  the  other 
on  May  18  (339,624).  On  the  decline  of  the  June  rise  of  this  year  a 
late  and  unusually  large  pulse  for  the  season  appears  (June  11)  at 
2,438,400.  An  examination  of  the  hydrograph  will  indicate  that 
almost  without  exception  these  pulses  attend  the  run-off  of  im- 
pounded backwaters  after  recent  invasion,  or,  as  on  April  29  and 
May  18,  after  a  temporary  check  in  the  run-off.  During  those 
times  when  the  channel  contributes  to  the  backwaters,  that  is,  dur- 
ing rising  floods,  Dinobryon  declines  in  numbers ;  and,  on  the  other 
hand,  it  reaches  its  greatest  development  in  channel  waters  during 
the  run-off  of  the  flood. 

In  1896-1897  the  interval  of  collection  (Pt.  I.,  Table  III.)  is 
again  too  great  to  trace  satisfactorily  the  fluctuations  of  Dinobryon. 
There  is  a  pulse  on  December  3  of  157,609  and  on  April  27  of 
172,800. 

In  1897-98  Dinobryon  appears  first  on  December  7,  with  a  pulse 
of  1,807, 200, during  a  period  of  low  water  and  ice  blockade  with  no 
backwater  contributions.  It  declines,  and  after  December  21  does 
not  again  return  until  March  22,  when  an  isolated  record  appears. 
The  vernal  pulse  begins  April  19  and  culminates  May  10  at  84,- 
841,600  on  the  declining  spring  flood  (Pt.  I.,  PL  XII.).  Dinobryon 
declines  at  once  during  a  fortnight  of  rising  water,  and  two  minor 
pulses  on  the  decline  of  the  flood — one  on  June  7  of  70,400  and  one 
on  June  28  of  219,840 — complete  its  vernal  cycle. 

The  hydrographic  conditions  in  1898-99  were  very  different  from 
those  of  the  preceding  season,  and  we  find  a  marked  change  in  the 
seasonal  occurrence  of  Dinobryon.  From  November  to  March 


76 

there  are  three  rises  to  overflow  stages  (Pt.  I.,  PL  XII.  and  XIII.) 
with  intervening  declines  of  a  month's  duration.  There  is  a  pulse 
of  Dinobryon  in  each  of  these  periods  of  declining  flood.  The  pulse 
of  275,200  on  December  20  follows  the  November  flood,  and  it  is 
followed  by  a  minimum  of  1,500  on  the  rising  flood  of  January  10. 
The  numbers  slowly  increase  until  a  meteoric  rise  on  February  7 
to  6,486,700  and  on  February  14  to  22,621,440  is  followed  again  by 
another  decline, to  25,920  on  February  28,  with  the  sudden  flood  of 
that  week.  During  the  .maximum  flood  stage  in  March  (Pt.  I.,  PL 
XIII.)  no  Dinobryon  was  recorded,  but  it  reappeared  again  on 
March  21.  The  suspension  of  our  plankton  operations  interrupted 
the  further  tracing  of  the  fluctuations. 

From  the  facts  above  detailed  it  is  very  evident  that  the  pulses 
of  Dinobryon  occur  in  channel  waters  at  times  when  the  run-off  of 
impounded  backwaters  is  making  its  greatest  contribution  to  the 
river  plankton.  These  are  times  of  greatest  stability  of  the  en- 
vironment in  all  respects  save  river  level  and  its  sequences.  The 
impounded  waters  have  come  from  regions  of  slight  current  and 
decaying  vegetation,  and  there  has  been  time  in  those  localities  for 
the  decay  of  sewage  and  debris,  and  for  the  growth  of  planktonts 
such  as  Dinobryon.  These  conditions  of  the  environment  are 
therefore  favorable  for  the  growth  pulses  of  Dinobryon.  The 
phenomenon  of  pulses  of  growth  is  not,  however,  to  be  considered 
as  merely  the  result  of  declining  floods.  These  afford  a  favorable 
environment  and  doubtless  determine  within  certain  limits  the 
time  and  the  extent  of  the  pulse.  The  phenomenon  is  one  common 
to  most  plankton  organisms,  and  occurs  in  Dinobryon  of  lakes  where 
floods  are  of  little  significance. 

Any  evidence  of  recurrent  minor  pulses  in  Dinobryon  at  brief 
intervals  is  lacking. 

Dinobryon  has  been  found  in  our  plankton  through  practically 
the  whole  range  of  temperatures,  but  it  disappears  when  maximum 
summer  heat  is  reached  and  does  not  return  until  the  water  cools 
to  45°  or  lower.  Large  pulses,  such  as  that  of  February  21,  1899 
(22,621,440),  have  developed  at  temperatures  approximating  32°, 
and  largely  under  the  ice.  The  vernal  pulse  of  April-May  has  been 
recorded  at  temperatures  ranging  from  60°  to  79°,  but  generally 
nearer  the  former.  No  well-defined  optimum  temperature  appears, 
and  the  seasonal  distribution  suggests  that  the  high  temperatures 


77 

of  our  summer  waters  are  inimical  to  Dinobryon.  That  its  absence 
from  the  plankton  at  that  time  is  not  due  merely  to  low- water  con- 
ditions is  shown  by  the  December  pulse  in  189 7, under  the  most  pro- 
nounced type  of  such  conditions. 

Dinobryon  is  a  common  planktont  in  the  Great-Xakes  (Kofoid, 
'95)  during  the  summer  months,  but  surface  temperatures  here 
rarely  exceed  68°,  and  are  10°  to  20°  below  those  of  the  Illinois 
River.  In  German  lakes  Apstein  ('96)  finds  the  maximum  develop- 
ment of  Dinobryon  in  June  and  a  continuance  through  the  summer 
in  reduced  numbers,  but  temperatures  are  also  10°  to  20°  (F.) 
lower  than  in  our  waters.  In  the  case  of  D.  stipitatum  there  is  a 
second  maximum  in  August.  Lauterborn  ('93)  finds  Dinobryon 
throughout  the  winter  in  the  plankton  of  the  Rhine,  with  a  maxi- 
mum in  April-May,  with  diminished  numbers  during  the  summer, 
and  a  second  maximum  in  September. 

The  filter-paper  collections  give  very  much  larger  numbers, 
owing  partly  to  the  inclusion  of  small  colonies  which  escape 
through  the  meshes  of  the  silk  net  in  the  usual  method  of  collec- 
tion. The  numbers  are  increased  at  least  thirty-fold  if  filter  col- 
lections are  utilized  instead  of  silk,  as  above. 

The  size  of  the  colonies  in  the  collections  varies  greatly,  the 
averages  ranging  from  three  to  forty-eight  cells.  The  maximum 
pulse  is  attended  or  followed  by  a  considerable  decrease  in  the  size 
of  the  colony.  In  the  pulse  of  February  21,  1899,  the  average  num- 
ber of  cells  in  the  colony  falls  from  thirteen  to  sixteen,  during  the 
rise  of  the  pulse,  to  seven,  at  its  culmination.  On  the  pulse  of 
May  10,  1898,  the  average  is  thirteen,  and  a  week  later,  when  the 
pulse  declines  from  16,153,600  to  43,200,  the  average  size  of  the 
colony  drops  to  three  cells.  Cysts  also  are  most  frequent  during 
and  subsequent  to  maximum  development.  Dinobryon  is  some- 
times covered  with  large  numbers  of  minute  choanoflagellates, 
probably  Salpingosca  minuta  Kent.  Frequently  colonies  occur  in 
which  only  the  younger  cells  are  alive. 

Dinobryon  is,  in  the  light  of  its  distribution,  one  of  the  impor- 
tant synthetic  planktonts  of  the  colder  months,  and  is  one  of  the 
primal  links  in  the  chain  of  food  relations  of  that  season,  serving 
as  food  for  some  of  the  winter  Cladocera  and  Copepoda.  The  fact 
that  its  maxima  frequently  occur  when  volumetric  minima  appear — 
as,  for  example,  on  February  21,  1899 — indicates  that  Dinobryon 


78 

does  not  directly  contribute  much,  even  at  its  maximum  develop- 
ment, to  the  volume  of  the  plankton  taken  in  the  silk  net.  On 
the  other  hand,  its  rapid  multiplication,  as  evidenced  by  its  meteoric 
pulses,  may  serve  to  build  up  a  more  permanent  and  bulkier  animal 
plankton,  and  thus  indirectly,  in  a  cumulative  way,  it  may  be  of 
considerable  quantitative  importance. 

The  inclusion  of  all  the  variants  of  Dinobryon  as  a  single  species 
has  been  favored  by  Wesenberg-Lund  ('00),  who  regards  D.  stipi- 
tatum  as  the  summer  form  of  D.  sertularia.  In  our  plankton,  D. 
stipitatum  has  occurred  sporadically  in  December  and  March,  but 
it  is  most  abundant  during  the  vernal  pulse  in  April-May.  Its 
distribution  thus  in  the  main  supports  that  author's  contention  in 
that  it  is  found  during  the  warmer  portion  of  the  seasonal  cycle  of 
Dinobryon  in  our  waters,  though  not  in  our  summer  plankton.  It  is 
not  desirable  in  this  connection  to  enter  further  into  a  discussion  of 
problems  which  have  been  raised  by  the  splitting  up  of  Dinobryon 
into  so  large  a  number  of  forms.  Lemmermann  has  found  seven- 
teen species  and  varieties  within  the  limits  of  the  subgenus  Eudino- 
bryon.  A  discussion  of  their  validity  involves  not  only  some  per- 
plexing problems  of  synonymy,  but  also  an  extensive  examination 
of  a  large  amount  of  material  showing  seasonal  changes,  and, 
above  all,  a  series  of  experiments  which  shall  demonstrate  the 
limits  of  variation  within  a  known  line  of  descent  and  in  the  sea- 
sonal range  of  environmental  conditions.  It  involves,  moreover, 
the  fundamental  question  of  the  criterion  of  species.  The  papers 
of  Lemmermann  ('00)  and  Brunnthaler  ('01)  have  appeared  since 
my  work  of  enumeration  was  completed.  I  recognize  among  the 
forms  which  they  have  sought  to  establish  the  following  which 
occur  in  our  plankton:  D.  sertularia  Ehrbg.,  D.  sertularia  var. 
thyrsoideum  (Chodat)  Lemm.,  D.  sertularia  var.  alpinum  Imhof, 
D.  protuberans  Lemm.,  D.  sociale  Ehrbg.,  D.  stipitatum  Stein,  D. 
stipitatum  var.  americanum  Brunn.,  D.  stipitatum  var.  bavaricum 
(Imhof)  Zach.,  D.  elongatum  Imhof,  D.  elongatum  var.  undulatum 
Lemm.,  D.  cylindricum  Imhof,  D.  cylindricum  var.  palustre  Lemm., 
D.  cylindricum  var.  schauinslandii  (Lemm.)  Lemm.,  D.  cylindri- 
cum var.  ped^forme  (Lemm.)  Lemm.,  D.  cylindricum  var.  diver  gens 
(Imhof)  Lemm.,  andD.  cylindricum  var.  angulatum  (Seligo)  Lemm. 

As  a  result  of  my  attempts  to  refer  all  of  the  individuals  which  I 
have  seen  in  my  work  of  enumeration  to  species,  I  am  of  the  opinion 


79 

that  we  are  dealing  in  the  case  of  the  species  of  Dinobryon  above 
cited  with  a  single  variable  organism,  whose  extremes  of  variation 
only  have  been  regarded  as  separate  species.  The  connecting  links 
are  sufficiently  abundant  still  and  the  union  of  several  types  in  a 
single  colony  is  sufficiently  frequent  to  lend  some  weight  to  my  con- 
clusions with  regard  to  those  forms  which  have  been  under  my 
observation.  In  the  interests  of  utility  as  well  as  in  the  interests 
of  well-grounded  taxonomy,  it  is  extremely  desirable  that  the 
establishment  of  new  species  among  variable  plankton  organ- 
isms should  be  attempted  with  extreme  caution  and  only  after  the 
fullest  study  of  the  range  and  conditions  of  variability.  The  insta- 
bility of  the  taxonomic  structures  which  Brunnthaler  and  Lemmer- 
mann  have  recently  raised,  is  evidenced  by  the  differences  in  syno- 
nymic, varietal,  and  specific  rank  given  to  the  variants  of  Dino- 
bryon by  these  two  systematists,  who  have  but  recently  mono- 
graphed the  group,  largely  if  not  wholly  from  the  systematic  point 
of  view.  The  changing  estimate  of  validity  which  Lemmermann 
himself  has  put  upon  his  own  species  or  varieties — for  example, 
schauinslandii,  pediforme,  and  curuatum — rgives  further  evidence 
that  the  basis  upon  which  they  rest  is  at  the  best  but  slight. 
It  is  my  firm  conviction  that  the  establishment  of  new  species 
among  the  organisms  of  the  plankton  of  fresh  water  can  be 
satisfactorily  accomplished  only  after  careful  analysis  of  the  limits 
of  variation  within  the  range  of  environmental  conditions.  Stand- 
ards less  comprehensive  than  this  can  yield  results  of  but  temporary 
or  local  value  and  can  lead  to  but  little  permanent  advance  in 
science,  and  they  bring  only  perplexity  and  chaos  where  order 
should  reign.  • 

Diplosiga  frequentissima  Zach.* — Average  number,  1,736,538. 
This  minute  flagellate  is  found  upon  the  rays  of  the  colonial  diatom 
Asterionella,  often  in  great  numbers  and  so  thickly  set  as  to  leave 
little  unoccupied  space.  It  was  found  in  each  year  at  the  time  of 
the  vernal  pulse  of  Asterionella  in  April-May,  and  was  as  a  rule  most 
abundant  immediately  after  the  maximum  growth  of  Asterionella 
had  been  attained.  Beyond  an  isolated  occurrence  in  January  it 
was  not  recorded  at  other  times  than  during  the  months  of  April 
and  May. 

Eudorina  elegans  Ehrbg. — Average  number,  14,362.  About 
twice  as  abundant  in  1897.  The  distribution  of  this  species  is 


80 

somewhat  erratic.  It  has  occurred  in  every  month  from  February 
through  October,  but  in  smaller  numbers  and  sporadically  in  the 
colder  months.  In  1898  its  seasonal  curve  is  of  characteristic  form. 
It  makes  its  appearance  March  1 5 ,  and  is  continuously  present  until 
the  end  of  September.  There  is  a  vernal  maximum  April  26  of 
240,000,  but  no  corresponding  autumnal  one.  In  1898  there  are 
indications  of  recurrent  pulses  at  brief  intervals  which  coincide  in 
location  immediately  or  approximately  with  similar  ones  of  Gonium 
and  Pandorina.  These  pulses  occur  March  IS  (3,600),  April  5 
(2,800),  April  26  (240,000),  June  14  (60,000),  August  2  (8,000), 
August  23  (3,200),  and  September  20  (2,000).  The  minima 
between  these  pulses  in  all  cases  but  one  fall  below  1,000.  In 
1897  a  vernal  pulse  was  not  detected,  a  maximum  of  496,000 
occurring  August  31,  and  but  three  mindr  pulses  appearing.  In 
1896  this  species  appeared  in  the  plankton  on  February  20, 
and  remained  until  the  end  of  August  with  a  month's  interrup- 
tion in  May- June.  There  were  no  marked  pulses,  exceeding 
15,000,  in  that  year.  The  absence  of  the  spring  flood  (Pt.  I., 
PI.  X.)  and  the  disturbed  hydrograph  of  the  summer  may  account 
for  this  suppression  of  development  in  Eudorina.  The  distribu- 
tion in  preceding  years  is  also  irregular. 

Eudorina  begins  its  seasonal  development  at  temperatures  but 
slightly  above  32°,  but  any  considerable  growth  is  not  attained 
until  at  least  45°  has  been  reached,  and  the  largest  pulses  on  record 
have  been  at  the  close  of  the  period  of  maximum  summer  heat  at  a 
temperature  of  80°,  and  the  vernal  pulses  have  been  at  60°  or  above. 
The  disappearance  of  Eudorina  from  the  plankton  in  the  early  fall, 
about  the  tim£  that  foliage  is  killed  by  autumnal  frosts,  has  been 
constant  in  the  different  years. 

Eudorina  is  not  sufficiently  abundant  to  be  of  any  considerable 
importance  in  determining  directly  the  volume  of  the  plankton. 
It  serves  as  food  for  many  of  the  rotifers,  and  is  itself  frequently 
parasitized  by  Dangeardia  mammillata  Schroder,  which  destroys  the 
cells  but  leaves  the  matrix  intact.  There  are  times  when  it  is 
hardly  possible  to  find  perfect  colonies,  and  when  it  is  not  unusual 
to  see  colonies  swimming  about  propelled  by  one  or  two  surviving 
cells. 

Euglena  acus  Ehrbg.* — Average  number,  214,807.  Found  from 
the  middle  of  March  till  the  first  of  November,  and  most  abundantly 


81 

in  late  summer  and  early  autumn.  It  escapes  through  the  silk  net 
readily,  and  no  marked  pulses  in  occurrence  appear  in  the  erratic 
data  of  the  filter-paper  collections.  It  is  found  in  the  water-bloom, 
and  is  predominantly  a  warm- water  planktont. 

Euglena  deses  Ehrbg. — Occurs  occasionally  in  the  plankton  and 
water-bloom  during  summer  months. 

Euglena  elongate  Schew.* — Average  number  in  1897,  278,970. 
It  is  found  irregularly  in  our  plankton  and  water-bloom  from  July 
to  October.  Originally  described  from  New  Zealand. 

Euglena  oxyuris  Schmarda.* — Average  number,  960,769.  Next 
to  E.  viridis  this  is  the  most  abundant  member  of  the  genus  in  our 
plankton.  It  is  abundant  during  the  summer,  especially  towards 
its  close  during  low-water  conditions,  when  the  water-bloom, 
in  whose  formation  it  shares,  is  best  developed.  There  is  no 
vernal  development,  and  the  fluctuations  are  but  slight  in  com- 
parison with  those  of  most  organisms  of  the  plankton:  There  is  a 
slight  indication  of  recurrent  pulses  at  intervals  of  a  few  weeks. 
Its  optimum  temperature  lies  near  that  of  maximum  summer 
heat,  that  is,  about  80°,  though  some  tendency  to  run  over  into 
autumn  months  is  manifest. 

Euglena  sanguinea  Ehrbg. — There  are  only  sporadic  occurrences 
of  this  species  in  the  plankton.  It  is  found  along  with  E.  vindis 
among  matted  growths  of  Lemnacea,  and  on  exposed  and  reeking 
mud  flats,  where  it  forms  patches  of  bright  red  color  often  of  large 
extent.  It  may  be  only  a  physiological  condition  of  E.  viridis, 
with  which  it  is  always  found.  It  has  appeared  in  the  plankton 
most  frequently  in  September,  though  found  elsewhere  throughout 
the  summer. 

Euglena  spirogyra  Ehrbg. — Found  but  once — in  October,  in  the 
river  plankton. 

Euglena  viridis  Ehrbg.* — Average  number,  1,571,731 ;  from  silk 
collections  only  8,653.  This  is  the  most  abundant  of  the  larger 
green  flagellates  in  our  plankton,  and  constitutes  the  greater  part  of 
the  water-bloom  of  summer  months,  when  it  forms  towards  four 
p.  m.  a  livid  green  scum  on  the  immediate  surface  of  the  water. 
Collections  of  the  silk  net  give  no  clue  to  its  abundance  and  shed  no 
light  on  its  seasonal  distribution.  The  filter-paper  collections  indi- 
cate its  presence  from  March  to  December,  but  in  numbers  only 
during  the  warmer  period,  from  May  to  October.  There  is  no  ver- 


82 

nal  pulse  though  there  are  slight  traces  of  minor  irregularities,  and 
on  September  7,  1897,  a  single  unusual  deyelopment  of  58,000,000. 
Its  optimum  temperatures  lie  close  to  the  maximum  heat  of  summer 
months.  It  is  found  not  only  in  water-bloom  and  plankton,  but 
also  along  shores,  on  mud  banks,  and  in  sequestered  pools  and  bays 
where  temperatures  reach  90°  and  over.  Lightly  colored  and  semi- 
transparent  individuals  of  this  and  other  species  of  the  genus  are 
found  frequently  in  the  plankton,  suggesting  an  approach  to  holo- 
zoic  nutrition  in  nature,  such  as  Zumstein  ('99)  has  demonstrated 
experimentally  in  E.  gracilis.  Euglena  is  quantitatively  one  of  the 
most  important  links  in  the  chain  of  food  relations  of  the  summer 
plankton,  converting  nutrient  matters  in  the  water,  both  organic 
and  inorganic,  into  food  for  the  Rotifer  a  and  Entomostraca  of  that 
season  of  the  year.  It  in  a  measure  replaces  the  diatoms,  some  of 
which  decrease  in  number  or  disappear  during  the  warmer  months. 

Glenodinium  cinctwn  Ehrbg.* — Average  number,  1,360,192. 
This  species  is  generally  present  from  the  middle  of  March  till  the 
end  of  September,  though  sporadic  occurrences  are  found  in  winter 
months.  There  is  a  pulse  on  March  29  of  4,260,000  at  a  temperature 
of  49°,  and  another  August  9  of  25,200,000  at  83°.  This  small 
planktont  usually  escapes  through  the  silk  net.  It  may  be  that 
several  species  have  been  included,  as  the  conditions  of  plankton 
enumeration  do  not  permit  close  scrutiny  of  such  small  organisms, 
lacking  prominent  structural  characteristics.  It  seems  to  be  a 
perennial  planktont  with  a  wide  range  of  temperature  adaptation, 
and  with  a  growing  period  approximating  that  of  the  land  flora  of 
our  latitude. 

Gonium  pectorals  O.  F.  Mull. — This  colonial  flagellate  has  been 
found  in  the  water-bloom  in  large  numbers,  especially  in  the  back- 
waters. It  was  taken  in  the  river  plankton  in  1897  and  1898  in 
May  and  again  in  August  and  September.  These  pulses  coincide  in 
location  with  those  of  Pandorvna  and  Eudonna. 

Lepocinclis  ovum  Ehrbg.* — Average  number,  401,538;  silk 
3,719.  This  species  appears  in  the  plankton  in  April  and  continues 
until  the  end  of  October,  with  sporadic  appearances  in  winter 
months.  There  is  no  vernal  pulse,  and  in  both  1897  and  1898  maxi- 
mum numbers,  43,200  and  50,400,  occur  at  the  height  of  midsummer 
heat  in  August.  In  both  years  there  are  well-defined  recurrent 
pulses  at  intervals  of  three  to  six  weeks  to  be  traced  in  the  silk 


83 

collections.  The  optimum  temperatures  plainly  lie  near  the  maxi- 
mum, that  is,  about  80°,  and  the  season  of  growth  approximates 
that  of  the  land  flora,  being  limited  to  the  months  of  April-Septem- 
ber. This  is  a  variable  organism,  and  a  number  of  species  have  been 
described  in  the  genus  in  recent  years.  Many  of  these" occur  in  our 
waters,  but  no  attempt  has  been  made  to  separate  them,  since  they 
are  based  on  minute  characters. 

Mallomonas  plosslii  Perty.  and  M.  producta  Zach. — These  two 
forms  will  be  treated  together,  as  in  my  opinion  they  are  merely 
divergent  variants — perhaps  seasonal— of  a  single  species.  In  1898 
M.  plosslii  was  found  but  three  times — in  June  and  July — and  M. 
producta  eight  times — from  May  through  September.  In  1897  the 
latter  only  was  recorded,  and  in  September  and  October.  In  1896 
M.  plosslii  appeared  in  July  and  M.  producta  in  April  and  August. 
In  1895  M.  producta  alone  was  recorded,  and  that  in  November. 
The  data  are  hardly  sufficient  for  generalization,  but  so  far  as  they 
go  they  indicate  that  producta  is  more  prevalent  in  late  summer  and 
autumn  and  plosslii  in  early  summer,  the  more  attenuate  form 
(producta}  in  the  warmer  season. 

Biitschli  ('80-' 89)  has  intimated  that  there  may  be  some  genetic 
connection  between  Mallomonas  and  Synura  uvella.  Certain 
features  of  its  occurrence  in  our  plankton  lend  their  support  to  this 
view.  Synura  in  our  waters  is  a  winter  planktont,  with  December 
and  February  or  March  pulses.  Mallomonas  is  a  summer  planktont, 
making  its  first  appearance  during  the  time  of  the  decline  of  Synura, 
and  when  many  of  the  colonies  of  the  latter  are  breaking  up  into 
their  individual  zooids.  Again,  the  differences  in  structure  and  size 
between  the  two  genera  are  quite  superficial,  and  might  result  from 
the  growth  attending  the  free  life  of  a  Synura  zooid  and  its  prepara- 
tion for  sporulation.  It  is  a  noticeable  phenomenon  that  the  pro- 
portion of  sporulating  individuals  of  Mallomonas  in  the  plankton  is 
exceptionally  large  among  all  plankton  organisms.  "Free  cells" 
of  Synura  are  plainly  referable  to  that  genus  by  their  resemblance, 
and  by  the  fact  that  they  are  often  united  in  clusters  of  several  indi- 
viduals forming  fragments  of  disintegrating  colonies.  It  may  be 
that  some  reproductive  phase,  as  conjugation,  intervenes  between 
the  free-cell  condition  of  Synura  and  the  Mallomonas  stage,  and 
that  the  relatively  smaller  numbers  of  the  latter  are  due  to  the  in- 
frequency  of  this  process.  While  the  features  of  seasonal  distribu- 

(7) 


84 

tion,  structure,  and  sporulation  thus  suggest  the  possibility  that 
Mallomonas  is  a  free  zooid  stage  leading  to  sporulation  in  Synura, 
they  do  not  demonstrate  it,  and  the  genera  must  stand  in  statu  quo 
until  breeding  experiments  shall  clearly  demonstrate  the  full  life- 
cycle  of  Synura. 

Pandorina  morum  Bory. — Average  number,  6,957.  In  1898  this 
organism  was  about  half  as  abundant  as  Eudorina,  but  in  1897  it 
more  than  equals  it.  On  account  of  the  small  size  and  the  motility 
of  the  colonies  many  of  them  escape  through  the  silk,  so  that  it  is 
not  so  adequately  represented  in  silk-net  collections  as  Eudorina. 
It  is  probably  the  most  important  quantitatively  of  the  Volvocidcz 
in  our  plankton.  It  occurs  from  April  to  October,  with  a  few  spo- 
radic appearances  in  March  and  up  to  January.  Its  greatest  growth 
occurs  from  May  to  October.  There  is  no  predominant  vernal  pulse 
in  1898,  but  a  series  of  smaller  ones  culminating  May  3  (48,400), 
June  14  (60,000),  July  26  (63,200),  and  August  30  (3,200), — all  upon 
declining  floods  (Pt.  I.,  PL  XII.)  and  coincident  with  pulses  of 
other  VolvocidcB — Eudorina  and  Gonium.  In  1897  its  seasonal  dis- 
tribution was  also  similar  to  that  of  these  genera,  exhibiting  a  max- 
imum pulse  August  31  of  638,000  at  80°.  In  1896,  a  year  of  inter- 
rupted hydrograph  (Pt.  I.,  PL  X.),  Pandorina  attained  no  marked 
development.  Its  optimum  temperatures  lie  at  and  above  60°, 
and  its  larger  pulses  appear  during  the  season  of  maximum  temper- 
ature, that  is,  at  about  80°.  Pandorina  does  not  attain  any  marked 
autumnal  growth,  but  declines  in  September,  and  as  a  rule  dis- 
appears in  October.  The  period  of  its  growth  thus  lies  within  that 
of  the  land  flora. 

As  in  Eudorina,  so  also  here,  parasitism  by  Dangeardia  mammil- 
lata  is  of  frequent  occurrence.  Pandorina  is  an  important  element 
in  the  food  of  summer  rotifers  such  as  Brachionus. 

Peridinium  tabulatuni  Ehrbg.* — Average  number,  3,875,769; 
silk,  3,711.  This  is  a  perennial  planktont,  having  been  found  in 
every  month  of  the  year.  Its  principal  development  is,  however, 
reached  during  warmer  months,  from  May  till  September.  In  1897 
the  maximum  pulse  of  172,800  was  on  August  10,  and  in  1898  one  of 
66,800  fell  on  July  26,  the  temperatures  being  81°  and  89°  respectively. 
The  only  exception  to  this  predominance  in  warm  months  is  an  iso- 
lated pulse  of  2,400  which  developed  on  the  declining  flood  of  Febru- 
ary, 1899  (Pt.  I.,  PL  XIII).  The  absence  of  any  autumnal  development 


85 

of  this  species  is  noticeable.  Its  optimum  temperatures  lie  close  to 
the  summer  maximum  (80°),  and  though  perennial,  its  occurrences 
at  other  seasons  than  late  spring  and  summer  are  irregular  and  its 
numbers  few.  Its  seasonal  distribution  in  German- lakes,  as  re- 
ported by  Apstein  ('96),  is  similar  to  that  in  the  Illinois  River. 
The  Peridiniidas  play  but  an  insignificant  part  in  the  plankton  of  the 
Illinois  River. 

Phacus  longicaudus  Ehrbg.* — Average  number,  61,153;  silk,  3 ,03 1 . 
This  species  in  1898  made  its  first  appearance  in  the  plankton  on  March 
23  and  continued  till  November  15.  The  species  is  small  enough  to 
escape  through  the  silk  net,  and  the  data  from  such  collections  do 
not  fully  express  its  seasonal  fluctuations.  There  is  no  marked 
vernal  pulse,  and  there  are  traces  of  but  a  few  small  ones  during  the 
summer,  the  largest  in  1898  being  one  of  35,200  on  September  27. 
The  distribution  in  previous  years  is  much  the  same.  A  well-sus- 
tained development  throughout  the  warmer  months — save  when 
rising  floods,  as  that  of  May,  1898,  reduce  the  numbers — indicates 
that  the  optimum  temperature  for  the  species  approaches  the 
summer  maximum  (80°).  There  are  almost  no  occurrences  below 
45°.  This  is  the  most  abundant  member  of  the  genus  in  our  plank- 
ton, but  it  is  not  quantitatively  an  important  element  therein. 

Phacus  pleur one ctes  Nitzsch.* — Average  number,  450,000;  silk, 
298.  It  is  less  abundant  (from  one  fifth  to  one  tenth)  than  P. 
longicauda  in  the  catches  of  the  silk  net  but  apparently  much  more 
abundant  in  the  filter-paper  collections,  which  may  be  due  in  part 
to  its  smaller  size  and  greater  tendency  to  escape  through  the  silk 
in  the  collections  of  the  net.  Its  occurrences  are  even  more  closely 
limited  to  summer  months — from  June  till  September.  There  is  no 
vernal  development,  and  the  largest  numbers  occur  during  the 
period  of  maximum  heat.  Pulses  are  but  feebly  defined.  It  is  also 
a  summer  planktont. 

Phacus  pyrum  Ehrbg.  was  found  but  once — on  August  10,  1897. 

Phacus  triqueter  Ehrbg.  occurred  in  small  numbers  during  July 
and  August,  1897. 

Platydorina  caudata  Kofoid. — Average  number,  17.  In  1898 
this  interesting  new  genus  of  the  Volvo cidoe  was  found  in  the  plank- 
ton only  in  the  latter  part  of  July.  In  1897  it  was  much  more  abun- 
dant (average  number,  21,963)  and  ranged  from  July  14  to  October 


86 

12.  There  was  a  pulse  on  July  21  of  18,400  and  another  on  Septem- 
ber 7  of  600,000.  In  previous  years  the  occurrences  were  scattering, 
but  confined  to  July,  August,  and  early  September.  It  is  evidently 
a  summer  planktont,  whose  optimum  temperature  lies  near  the  max- 
imum attained  by  our  waters.  No  record  of  occurrence  below  60° 
was  made.  The  smaller  and  younger  colonies  escape  readily 
through  the  silk  net.  Its  pulses  in  1897  coincide  very  closely  with 
those  of  Gonium,  Pandorina,  Eudorina,  and  Pleodorina. 

Pleodorina  calif  arnica  Shaw. — Average  number,  11.  In  1897 
this  species,  in  common  with  other  members  of  the  family,  was  much 
more  abundant  than  in  any  other  year  of  our  work,  stable  conditions 
of  low  water  with  the  accompanying  sewage  contamination  seeming 
to  favor  its  development.  The  earliest  record  for  P.  calif ornica 
in  the  plankton  is  May  18,  1896,  at  71°.  This  was  a  year  of  low^er 
water  and  higher  temperatures  than  usual  in  spring  months  (Pt.  I., 
PI.  X.).  In  other  years  P.  calif  ornica  did  not  appear  until  June  or 
July.  It  continues  into  September,  the  latest  record  in  1895  being 
October  2.  In  1897  there  were  pulses  on  July  21  (5,600)  and 
September  7  (4,000).  The  occurrences  at  other  seasons  are  too 
scattered  to  trace  the  seasonal  fluctuations,  but  there  is  a  well-de- 
fined predominance  during  the  period  of  maximum  heat.  This  is 
evidently  a  summer  planktont,  whose  optimum  temperature  lies 
near  80°. 

Pleodorina  illinoisensis  Kofoid. — Average  number,  6,91  7  in  1897. 
This  is  somewhat  more  numerous  than  the  preceding  species,  and 
its  range  of  occurrences  is  quite  similar.  Its  maximum  pulse  in 
1897  (180,000)  is  on  August  31,  a  week  earlier  than  in  other  members 
of  the  family.  These  pulses  of  the  VolvocidcB  occur  (Pt.  I.,  PI. 
XLIV.)  in  a  depression  of  nitrates  and  just  prior  to  the  volumetric 
pulse  of  September,  1897.  This  pulse  is  doubtless  built  up  partly 
at  their  expense.  Their  decline  in  numbers  corresponds  with  its 
rise.  This  is  also  a  summer  planktont,  and  was  not  recorded 
below  71°. 

Salping&ca  brunnea  Stokes.* — This  species  was  not  recorded  in 
1898.  Average  number  in  1897,  1,887,356.  It  occurred  on  May 
25  and  July  21,  dates  of  culmination  of  pulses  of  Melosira  granulata 
var.  spinosa.  In  August-September  a  pulse  occurs,  culminating 
September  7  at  47,250,000 — a  week  after  the  culmination  of  a  Mel- 
osira pulse.  In  1896  (silk  collections  only)  it  was  present  through- 


87 

out  most  of  the  summer,  attending  only  approximately  the  sup- 
pressed and  interrupted  pulses  of  Melosira  in  that  year  of  disturbed 
hydrograph.  It  has  been  recorded  from  the  latter  part  of  April 
till  the  middle  of  September,  and,  as  a  rule,  above  60°.  This  beau- 
tiful little  choano flagellate  is  sessile  upon  the  filaments  of  Melosira, 
principally  upon  the  variety  spinosa,  and  but  rarely  upon  M.  varians 
or  other  planktonts  such  as  Pediastrum.  It  is  often  associated 
with  Bicosceca  lacustris  and  is  usually  found  upon  the  sides  of  the 
filaments,  the  bowl  of  the  transparent  brownish  lorica  being  closely 
sessile  upon  the  diatom.  In  one  instance  a  lorica  was  found  upon 
the  corner  at  the  end  of  the  filament.  The  lorica  had  adapted 
itself  to  this  novel  situation  by  an  angular  indentation  fitted  upon 
the  corner  of  the  diatom. 

Syncrypta  volvox  Ehrbg. — Average  number,  625.  This  species 
has  a  definite  and  somewhat  unusual  seasonal  distribution.  In 
1898  it  was  found  from  March  1  to  April  12,  and  reappeared  Novem- 
ber 8,  attaining  a  maximum  of  13,500  on  December  6,  and  of  43,000 
on  January  1,  declining  then  to  800  and  rising  on  February  14  to 
4,800,  and  subsequently  disappearing  in  the  flood  waters  of  March. 
It  was  not  recorded  in  1897.  In  1895  it  appeared  September  27  and 
continued  for  a  month,  reappearing  in  February  and  March,  and 
not  occurring  after  April  10.  It  has  attained  its  largest  develop- 
'ment  at  minimum  temperatures  under  the  ice — 43,000  January  3, 
1899,  at  32.7°.  The  greater  part  of  its  occurrences  in  1898-1899 
lie  very  near  this  temperature,  and  but  three  in  all  the  years  lie 
above  50°.  It  is  par  excellence  a  winter  planktont,  or  at  least  a 
cold-water  one. 

Its  occurrences  in  1895-1896  lie  near  the  beginning  and  the 
close  of  the  seasonal  pulse  of  Synura.  In  1898-1899  the  pulses  of 
Syncrypta  coincide  in  location  with  or  immediately  follow  those  of 
Synura.  The  resemblance  of  Syncrypta  to  small  colonies  of 
Synura  is  striking,  and  this  fact  combined  with  the  relation  of  their 
seasonal  fluctuations  raises  the  query  if  Syncrypta  may  not  be  an 
encysting  stage  of  the  Synura  colony.  Its  life  history  should  be 
fully  worked  out. 

Synura  uvella  Ehrbg. — Average  number  of  colonies,  8,463. 
The  seasonal  distribution  of  this  chrysomonad  flagellate  is  some- 
what similar  to  that  of  its  near  relative  Syncrypta.  It  is  a 
perennial,  though  predominantly  cold-water,  planktont.  It  appears 


88 

in  the  December  plankton  of  1894,  but  was  exterminated  from  the 
channel  plankton  taken  in  the  following  February  by  the  stagna- 
tion attending  the  long-continued  ice  blockade.  It  reappears  in 
April,  and  again  disappears  promptly,  but  does  not  return  until 
September  12,  and  not  in  numbers  until  October.  There  are  pulses 
November  20  (506,800)  at  42.8°,  and  December  30  (362,520)  at  36.5°. 
The  December  pulse  is  followed  by  a  decline,  with  a  rise  during 
February  to  a  well-sustained  maximum  during  March,  approaching 
400,000,  and  at  from  35°  to  48°.  The  decline  follows  in  April,  and 
there  are  only  isolated  occurrences  in  small  numbers  at  irregular 
intervals  during  the  summer.  Continuous  occurrence  begins  again 
in  September,  and  numbers  rise  rapidly  in  October.  There  is  a  pulse 
of  542,699  on  December  3  at  32.2°,  and  another  on  March  22,  1897, 
of  159,500  at  43.8°.  Synura  is  very  rare  indeed  in  the  summer  of 
1897,  and  in  the  prolonged  low  water,  sewage  contamination,  and 
higher  temperatures  of  the  unusual  autumn  of  that  year  it  does 
not  reappear  continuously  until  October  26,  at  59°,  and  does  not 
exceed  1,000  until  December  7,  at  32°.  There  is  a  low  maximum 
of  98,700  on  December  14  at  36°,  followed  by  a  decline  during  the 
rising  flood  of  January-March,  1898.  The  slight  cessations  in  the 
flood  invasion  (Pt.  I.,  PL  XII.)  in  January  and  in  the  second  weeks 
of  February  and  March  produce  prompt  responses  in  immediate 
rise  in  numbers  in  Synura.  Finally,  a  low  maximum  of  320,600  is 
attained  upon  the  crest  of  the  March  flood,  on  the  29th,  at  49°. 
This  is  followed  by  a  decline  during  April  and  a  few  scattered 
appearances  during  the  summer.  Synura  returns  at  the  end  of 
October  and  rapidly  mounts  to  a  pulse  of  1,999,500  on  November 
29  at  35°  with  the  first  decline  of  the  November  overflow  (Pt.  I., 
PI.  XII.).  A  second  pulse  of  2,764,800  on  December  20  at  33°, 
under  the  ice,  gives  way  to  a  decline  to  51,600  towards  the  end  of 
January,  1899,  during  rising  water.  On  February  14  another  pulse 
(348,800)  appears  at  32.5°,  under  heavy  ice,  and  declines  again  in 
the  sudden  flood  of  the  last  days  of  February,  but  recovers  quickly 
with  a  maximum  pulse  of  898,800  on  March  7  at  32.8°.  Within  a 
fortnight  this  falls  to  the  low  level  of  9,800,  but  its  further  history 
was  not  followed. 

From  these  data  it  is  evident  that  in  our  waters  at  least  Synura 
is  limited  to  the  months  from  October  to  April,  except  isolated  and 
irregular  occurrences  of  small  numbers  during  the  summer.  Its 


89 

optimum  temperatures  lie  below  50°,  and  its  greatest  development 
has  taken  place  in  minimum  temperatures  under  the  ice.  Rising 
floods  and  disturbed  hydrographies  conditions  tend  to  reduce  its 
mimbers  or  to  suppress  its  development,  while  declining  floods 
initiate  increase  in  numbers  and  favor  the  appearance  of  pulses. 
A  "late"  autumn  delays  the  appearance  of  Synura. 

Not  only  are  colonies  of  Synura  found  in  the  collections,  but  at 
times  large  numbers  of  free  cells  make  their  appearance.  These 
are  released  by  the  breaking  up  of  colonies,  and  occur  in  all  degrees 
of  isolation.  It  seems  to  be  a  natural  phenomenon,  and  occurs 
most  abundantly  with  or  immediately  after  the  crest  of  the  pulse. 
Thus  the  pulse  of  December  29  (1,999,500  colonies)  was  attended 
by  21,600,000*  free  cells  on  that  date.  A  week  later  there  were 
1,693,500  colonies  and  57,600,000  free  cells.  There  are  in  the  rec- 
ords several  instances  of  meteoric  increases  of  free  cells  at  other 
times  than  at  those  of  apparent  pulses.  It  does  not  seem  possible 
from  the  data  at  hand  to  determine  whether  this  is  due  to  environ- 
mental influences  or  to  the  accidents  of  collection  and  subsequent 
handling.  In  the  discussions  of  Mallomonas  and  Syncrypta,  sugges- 
tions have  been  made  that  these  organisms  may  be  stages  in  the 
life  cycle  of  Synura.  Synura  is  the  largest  and  by  far  the  most 
important  synthetic  organism  of  the  winter  plankton.  It  shares 
appreciably  in  the  winter  volumetric  pulses — as,  for  example,  those 
of  December,  1898  (Pt.  I.,  PI.  XII.). 

Its  fluctuations  do  not  seem  to  produce  any  marked  effect  upon 
the  nitrates,  possibly  because  the  latter  are  present  in  excess  of  the 
needs  of  Synura.  In  the  winter  of  1898  nitrates  are  high,  1.25 
parts  per  million  with  the  pulse  of  1,999,500  colonies  on  November 
29,  but  decline  rapidly  to  .1  on  December  13  with  a  fall  of  Synura 
to  78,000.  On  December  20, Synura  rises  to  2,764,800,  but  the  ni- 
trates rise  only  to  .35.  It  is  evident  that  the  nitrates  are  not  the 
only  factor  regulating  the  fluctuations  of  Synura. 

Marsson  ('00)  reports  Synura  as  abundant  in  the  winter  plankton 
of  lakes  about  Berlin,  and  Brunnthaler  ('00)  finds  it  in  the  winter 
plankton  of  the  Danube.  There  is,  however,  no  recorded  instance 
in  which  Synura  forms  so  prominent  a  part  of  the  plankton  of  a  body 
of  water  as  it  does  of  that  of  the  Illinois  River.  It  may  be  that  a 
closer  analysis  than  has  yet  been  given  the  potamoplankton  of  other 
streams  will  reveal  its  prominence  there  also.  It  is  present  (Kofoid 


90 

'95)  in  the  summer  plankton  of  the  Great  Lakes  at  temperatures 
15°  to  20°  below  the  summer  maximum  of  the  Illinois  River. 

Trachelomonas  acuminata  Schmarda.* — Average  number,  1,094,- 
615  ;  silk,  873.  This  species  appears  in  the  plankton  in  April  or  May 
and  continues  into  October  or  November.  There  is  no  vernal  pulse, 
and  the  data  are  too  irregular  to  trace  the  seasonal  fluctuations. 
The  greater  numbers  occur  during  the  period  of  maximum  heat. 
Excepting  a  single  occurrence  in  February,  this  species  has  been 
found  only  above  40°,  and  its  period  of  continuous  appearance  from 
May  to  October  lies  above  60°.  It  is  evidently  a  summer  plank- 
tont. 

Trachelomonas  hispida  Stein.* — Average  number,  1,002,115  ;  silk, 
1,251.  This  is  a  perennial  organism,  found  in  every  month  of  the 
year  but  in  larger  numbers  during  the  warmer  months.  It  was 
more  abundant  than  usual  in  the  winter  of  1897-98  following  the  low 
water  and  unusual  development  of  the  previous  fall.  There  are  no 
large  pulses  in  1898,  but  in  1897  there  is  indication  of  a  vernal  max- 
imum on  April  27  and  an  autumnal  one  of  85,500,000  on  September 
7.  The  data  are  too  irregular  to  trace  the  seasonal  fluctuations  in 
detail.  There  is  no  doubt,  however,  from  the  evidence  at  hand  that 
this  is  a  predominantly  warm-water  planktont  similar  to  the  other 
members  of  the  genus. 

Trachelomonas  volvocina  Ehrbg.* — Average  number,  17,672,692; 
silk,  7,162.  This  is  the  most  abundant  species  of  the  genus  and  is 
found  throughout  the  year  in  almost  every  collection.  It  is  most 
abundant  from  May  to  October,  during  the  period  of  maximum 
heat.  There  are  no  well-defined  vernal  or  autumnal  pulses,  but 
recurrent  maxima  during  the  summer  are  to  be  found  in  both  1897 
and  1898.  There  are  four  such  pulses  in  the  former  year,  and  in  the 
latter  five,  as  follows:  May  17  at  64°  (14,400,000), 'june  21  at  77° 
(147,600,000),  July  19  at  84°  (86,400,000),  August  9  at  83° 
(252,000,000),  and  October  4  at  71.5°  (11,700,000).  The  periods 
of  greatest  growth  thus  lie  above  60°  and  the  optimum  is 
near  80°.  None  of  these  pulses  coincides  with  a  volumetric 
maximum  of  the  silk-net  catches  (Pt.  I.,  PL  XII.).  They 
usually  follow  these  maxima  at  intervals  of  one  or  two  weeks — 
a  phenomenon  often  observed  in  other  synthetic  species.  It  may 
be  explained  by  the  decrease  in  animals  which  feed  upon  the  organ- 
isms in  question.  These  volumetric  pulses  are  predominantly 


91 

animal  in  their  composition,  and  when  they  decline  the  organisms 
upon  which  the  disappearing  animals  were  feeding  have  an  oppor- 
tunity to  multiply  with  less  decimation  in  their  ranks. 

This  species  is  one  of  the  most  abundant  of  the  synthetic  organ- 
isms in  the  summer  plankton,  and  next  to  Euglena  is  the  foremost 
among  the  synthetic  elements  of  the  food  cycle  of  the  plankton. 
The  presence  of  many  light-colored  or  even  colorless  forms  (forma 
hyalina  Kl.)  justifies  the  suspicion  that  members  of  this  genus,  like 
those  of  its  near  relative  Euglena,  adopt  holozoic  nutrition  in  the 
presence  of  abundant  organic  matter  suitable  for  food. 

This  species,  as  well  as  the  others  above  listed,  is  exceedingly 
variable  in  the  proportions  of  the  lorica,  in  its  color,  and  in  the 
development  of  the  neck.  It  is  very  desirable  that  its  life  history 
and  the  full  limits  of  its  variation  be  determined  before  many  more 
new  species  are  proposed  in  the  genus. 

In  addition  to  the  forms  above  listed,  the  following  have  been 
noted  as  present  in  small  numbers  in  the  summer  plankton,  viz. :  T. 
armata  Ehrbg.,  T.  caudata  Ehrbg.,  T.  torta  Stokes,  T.  urceolata 
Stokes,  and  T.  volvocina  var.  rugulosa  Kl. 

Uroglena  americana  Calkins. — This  species  was  found  in  small 
numbers  in  July  and  September,  1897,  and  in  January,  1899. 

Uroglena  radiata  Calkins. — This  species  was  found  in  January, 
1896 ;  in  April  and  May,  1897  ;  and  in  March  and  April,  1898.  There 
was  a  vernal  pulse  of  15,279  on  April  29,  1896. 

Uroglena  volvox  Ehrbg. — This  species  was  found  sparingly  in  the 
spring  plankton  in  1896.  Uroglena  is  one  of  the  few  organisms 
which  the  usual  method  of  plankton  collection  and  preservation 
fails  to  keep  in  fair  condition  for  subsequent  indentification. 
The  gelatinous  matrix  is  easily  crushed,  and  debris  adheres  to  it  so 
as  to  obscure  it  beyond  recognition.  Judging  from  the  frequency 
of  Uroglena  in  the  living  plankton  it  is  very  probable  that  the  genus 
is  much  more  abundantly  represented  in  the  Illinois  River  than  the 
data  at  hand  indicate.  The  genus  seems  to  prefer  the  cooler  waters 
of  autumn  and  spring  to  those  of  midsummer. 

Volvox  aureus  Ehrbg. — This  species  was  found  from  March  to 
August,  but  in  small  numbers  and  irregularly. 

Volvox  globator  L. — This  was  somewThat  more  abundant  than  the 
previous  species,  and  was  found  more  frequently,  especially  during 


92 

1895  and  1896.  It  occurred  from  the  first  of  May  till  the  end  of 
August,  but  always  in  small  numbers.  It  is  occasionally  abundant 
in  backwaters  where  there  is  much  vegetation. 

In  addition  to  the  Mastigophora  above  listed  there  were  many 
individuals  belonging  to  unidentified  species .  They  were  as  a  rule  the 
smaller  forms,  which  are  not  readily  identified  in  preserved  material 
and  under  the  conditions  of  plankton  enumeration.  They  consti- 
tute about  twenty-six  per  cent,  of  the  total  Mastigophora  enu- 
merated. In  silty  planktons  their  number  is  relatively  somewhat 
larger  on  account  of  the  difficulties  attending  the  determination  of 
species  in  such  material.  These  unidentified  flagellates  occur  in 
every  collection,  and  are  somewhat  more  abundant  in  the  summer 
months. 

RHIZOPODA. 

Average  number,  55,364,  including  filter-paper  collections; 
23,826  without  them.  This  group  of  Protozoa  is  numerically  of  less 
importance  than  the  ciliates  or  flagellates,  but  its  quantitative 
significance  is  greater  than  the  numbers  of  individuals  indicate. 
This  is  due  to  the  relatively  large  size  of  the  Rhizopoda,  and  also  to 
the  fact  that  plankton  collections  afford  only  an  irregular  and  in- 
complete record  of  the  rhizopodan  fauna  of  any  body  of  water,  and 
give  but  an  imperfect  idea  of  the  part  which  these  organisms  play 
in  the  total  economy  of  the  lake  or  stream.  This  results  from  the 
fact  that  they  are  as  a  rule  largely  bottom  or  shore-loving  species, 
and  are  generally  either  adventitious  or  temporary  constituents  of 
the  plankton. 

The  seasonal  distribution  of  the  total  Rhizopoda  in  the  Illinois 
River  gives  evidence  of  the  adventitious  or  temporary  nature  of  the 
contributions  of  the  group  to  the  plankton.  There  are  pulses  in 
1898  on  January  25  (66,388),  February  22  (141,524),  August  23 
(36,800),  September  27  (59,200),  and  November  15  (42,000),  all  of 
which  appear  on  rising  water  and  are  largely  adventitious,  their 
presence  in  the  plankton  being  due  to  the  disturbances  of  currents, 
waves,  and  the  like.  There  are  pulses  on  May  10  (49,800),  June  28 
(37,000),  and  July  19  (28,800)  which  cannot  be  traced  to  any 
general  hydrographic  condition.  These,  as  will  be  suggested  in  the 
discussion  of  the  seasonal  fluctuations  of  individual  species,  are 
probably  due  to  the  temporary  adoption  of  a  limnetic  habit  on  the 


93 

part  of  some  of  the  rhizopods,orto  the  appearance  of  limnetic  forms, 
varieties,  or  species — according  to  the  systematic  value  placed  upon 
these  eulimnetic  individuals.  I  am  inclined  myself  to  regard  them 
as  seasonal  forms  of  species  which  are  predominantly  of  the  bottom 
or  littoral  fauna,  which  have  multiplied  rapidly  under  the  stimulus 
of  abundant  food.  Owing  to  this  fact,  to  the  storage  in  their 
tissues  of  the  products  of  metabolism,  such  as  gas  and  oil  vacuoles 
which  tend  to  lighten  their  specific  gravity,  and  to  the  frailer 
structure  of  their  shells  under  conditions  of  rapid  multiplication, 
they  abandon  their  customary  benthal  or  littoral  habitat  and  assume 
temporarily  a  limnetic  distribution  in  the  plankton  where  they  con- 
tinue to  find  abundant  food.  Their  appearance  here  under  these 
circumstances  is  a  result  of  their  physiological  condition,  and  with 
its  cessation  they  decline,  as  shown  by  their  pulse-like  occurrences. 
Whatever  the  systematic  valuation  placed  upon  these  limnetic 
forms  may  be,  there  is  no  doubt  of  their  occurrence.  They  have 
appeared  in  every  year  of  our  operations,  but  were  most  prevalent 
in  1897,  a  year  of  most  stable  conditions,  and  also  in  the  quieter 
backwaters,  and  on  the  declining  spring  flood  or  June  rise  when  hydro- 
graphic  conditions  are  less  catastrophic  than  those  of  early  flood 
stages.  In  1897  there  was  a  pulse  of  68,400  (silk-net  only)  on  August 
8  and  another  of  1,268,400  on  September  7,  both  in  stable  conditions 
and  almost  exclusively  of  limnetic  types,  differing  in  this  respect 
from  the  pulse  of  141,524  on  February  22,  1898,  which  was  pre- 
dominantly of  an  adventitious  character,  resulting  from  the  flood 
of  that  period  (Pt.  I.,  PI.  XII.).  The  contrast  in  the  numbers  of 
Rhizopoda  in  the  plankton  during  warm  and  cold  seasons  of  the 
year  is  very  striking  in  1897.  The  average  per  m3,  per  collection 
from  May  1  to  October  1,  that  is,  above  60°,  is  161,045,  omitting  all 
filter-paper  collections,  while  in  the  seven  months  of  lower  tempera- 
tures this  average  is  only  4,771.  During  the  warmer  period  the 
June  rise  was  the  only  hydrographic  disturbance  (Pt.  I.,  PI.  XL) 
to  which  any  adventitious  increase  might  be  attributed.  This  con- 
trast is  less  evident  in  1898,  when  the  summer  hydrograph  was  more 
disturbed.  These  larger  numbers  during  warmer  months  may  be 
attributed  in  part  to  the  greater  numbers  of  the  Rhizopoda  in  their 
littoral  habitat,  and  in  part,  doubtless,  to  the  fact  that  at  low  water 
the  shore  and  bottom  fauna  are  brought  into  more  intimate  relation 
with  the  plankton,  and  in  the  river  the  disturbance  of  these  regions 


94 

by  current,  waves,  seines,  boats,  and  fish  make  relatively  larger 
contributions  at  low-water  stages  to  the  diversification  of  the 
plankton.  In  addition  to  these  factors,  however,  there  is  abun- 
dant indication  that  many  individuals  assume  during  the  warmer 
months  a  eulimnetic  habit,  and  that  some  of  the  Rhizopoda 
become,  for  the  time  being  at  least,  typical,  though  temporary, 
planktonts. 

It  naturally  follows  that  in  so  far  as  the  plankton  is  concerned, 
the  Rhizopoda  exhibit  a  seasonal  preference  for  the  warmer  months 
above  60°!  Maximum  numbers  were  attained  only  at  the  higher 
temperatures  save  in  those  instances  where  they  attend  winter 
floods.  In  a  measure  the  seasonal  distribution  of  the  Rhizopoda  in 
the  plankton  reflects  that  of  the  group  in  its  normal  habitat ;  but  at 
the  best  the  picture  is  incomplete. 

The  Rhizopoda  have  important  relations  in  the  economy  of  the 
plankton.  They  feed  upon  diatoms,  desmids,  the  smaller  algae,  and 
even  the  chlorophyll-bearing  Mastigophora  such  as  Trachelomonas 
and  Carteria.  Their  occurrences  in  the  plankton  do  not  exhibit  any 
striking  correlation  with  those  of  the  groups  named.  The  great 
pulse  of  September  7,  1897,  for  example  (PI.  II.),  lies  in  a  depression 
of  the  diatoms  and  coincides  with  pulses  of  Chlorophycece  and 
Mastigophora,  and  that  of  August  10  (68,400)  exhibits  a  similar 
relation,  the  diatoms  rising  the  following  week  as  the  Rhizopoda 
fall.  In  1898  the  pulse  of  Rhizopoda  on  June  28  of  37,000  (Table  I.) 
culminates  a  fortnight  after  that  of  the  diatoms  and  Chlorophycece 
and  a  week  after  that  of  the  Mastigophora.  It  thus  is  intercalated 
between  the  June  and  July  pulses  of  these  chlorophyll-bearing 
organisms  (PI.  II.).  The  Rhizopoda  pulse  of  July  19  (28,800),  on 
the  other  hand,  occurs  with  the  coincident  pulses  of  the  three 
groups  named  (PL  II.).  The  immediate  diluent  effect  of  flood 
waters  upon  the  plankton  combined  with  their  tendency  to  increase 
the  number  of  adventitious  Rhizopoda  results  at  times  in  the 
intercalation  of  their  pulses  with  those  of  the  chlorophyll-bearing 
organisms  whose  relative  numbers  are  reduced  by  the  dilution.  The 
data  evidently  do  not  afford  any  adequate  solution  of  the  inter- 
calations of  the  Rhizopoda  with  other  organisms. 

The  Rhizopoda  are  very  frequently  found  in  the  digestive  tract 
of  limnetic  rotifers,  but  I  have  never  noted  the  Entomostraca  feed- 
ing upon  them.  They  are  important  elements  in  the  food  of  young 


95 

fish  (Forbes,  '80)  such  as  the  Catostomidce  and  some  of  the  Silu- 
rida  and  minnows.  I  have  found  them  in  great  abundance  in  the 
intestine  of  the  adult  gizzard-shad  (Dorosoma),  and  in  the  contents 
of  the  digestive  tract  of  the  German  carp  (Cyprinus  carpio). 

In  the  pages  which  follow,  the  seasonal  distribution,  or  occur- 
rence in  the  plankton,  of  thirty-one  Rhizopoda  is  discussed,  and 
the  presence  in  the  plankton  of  the  Illinois  of  twenty-eight  other 
rhizopodan  forms  which  have  been  recognized  by  other  writers  as 
of  specific  rank  is  noted.  This  by  no  means  exhausts  the  rhizopo- 
dan fauna  of  the  environment  which  was  the  field  of  this  investiga- 
tion. A  continued  study  of  the  plankton  itself  would  doubtless 
greatly  extend  the  list  of  adventitious  forms  from  the  shore  and 
bottom,  and  a  more  careful  analysis  of  the  variants,  especially  in 
the  Difflugia  globulosa-lobostoma  group,  would  still  further  increase 
the  richness  of  the  fauna  from  the  systematic  point  of  view.  Hem- 
pel  ('99)  lists  sixteen  species  from  this  locality,  and  Penard  ('02), 
in  discussion,  remarks:  "Une  pareille  pauvrete  dans  une  region 
riche  en  organismes  de  toute  nature,  est  une  impossibilitie  mat  6- 
rielle."  However,  neither  Hempel's  paper  nor  the  present  one 
pretends  to  give  a  full  account  of  all  the  Rhizopoda  of  the  region. 
He  dealt  largely  with  plankton  collections,  and  the  present  paper 
deals  with  them  exclusively. 

There  is  but  little  in  plankton  literature  which  gives  with  any 
fulness  the  seasonal  distribution  of  the  Rhizopoda,  or  indicates  that 
they  are  of  any  considerable  importance  in  the  economy  of  the 
plankton.  The  importance  which  they  acquired  in  the  plankton  of 
the  Illinois  is  no  doubt  in  part  due  to  the  nature  of  the  environ- 
ment with  which  we  are  dealing.  The  somewhat  sporadic  and 
meteoric  character  of  their  appearances  in  our  waters  leads  to  the 
inference  that  full  seasonal  analyses  of  the  plankton  of  other  bodies 
of  water  at  brief  intervals  may  reveal  a  greater  prevalence  of  the 
Rhizopoda  in  the  plankton  than  has  hitherto  been  detected. 

DISCUSSION    OF    SPECIES    OF    RHIZOPODA. 

Amoeba  Umax  Duj. — This  was  frequently  abundant  in  the  water- 
bloom  of  midsummer,  but  was  not  identified  in  the  plankton 
collections. 

Amoeba  proteus  Rosel. — Average  number,  342.  The  individuals 
here  assigned  to  A.  proteus  include  those  taken  in  our  plank- 


96 

ton  which  belong  to  the  type  of  A.  radiosa  Ehrbg.,  a  type  which 
presents  no  distinctions  sufficiently  well-defined  to  separate  it  spe- 
cifically from  the  first-named  form.  It  seems  probable  that  A. 
radiosa  includes  small  individuals  of  A.  proteus  which  are  not,  at 
the  time  of  observation,  creeping  upon  a  substratum ;  that  is,  they 
are  limnetic,  floating  free  with  filamentous  pseudopodia  character- 
istic of  that  condition.  Verworn  ('97)  has  shown  that  A.  proteus 
takes  the  radiosa  form  in  weakly  alkaline  solutions.  Pond  water 
rich  in  algae  may  have  an  alkaline  reaction  (Knauthe,  '98)  in  bright 
sunlight.  Larger  individuals,  distinctly  referable  to  the  A.  proteus 
type  when  taken  in  the  plankton,  possess  at  times  the  slender  pseu- 
dopodia of  the  A.  radiosa  type  as  well  as  the  blunter  ones  charac- 
teristic of  the  A.  proteus  form.  I  see  no  valid  reason  for  separating 
the  two  as  distinct  species.  Most  of  the  Amoeba  recorded  from  the 
plankton  collections  belong  to  the  A.  proteus  type,  the  smaller  ones 
belonging  to  the  radiosa  type  probably  escaping  through  the 
meshes  of  the  silk  net. 

This  species  wTas  found  in  30  of  the  180  collections  examined, 
being  observed  in  all  months  of  the  year  except  May,  November, 
and  December.  The  conditions  attending  its  occurrence  suggest 
that  it  is  not,  habitually  at  least,  an  active  planktont  at  all  seasons 
of  its  occurrence,  but  rather  a  tycholimnetic  member,  an  invader 
from  the  littoral  or  bottom  fauna,  or  a  temporary  accession  during 
the  warmer  months.  In  the  first  place,  both  the  number  of  occur- 
rences and  the  numbers  of  individuals  found  are  small,  and  the 
seasonal  distribution,  plotted  from  the  data  of  the  collections  of 
the  five  years,  is  exceedingly  irregular.  Furthermore,  17  of  the  30 
occurrences  happened  on  rising  floods,  when  the  fauna  of  the  bot- 
tom and  shore  of  both  the  river  and  its  tributaries  is  most  mingled 
with  the  plankton.  Further  evidence  of  the  agency  of  floods  in 
introducing  Amceba  into  the  plankton  is  brought  to  light  by  a  com- 
parison of  its  occurrences  in  1897  and  1898.  As  shown  by  Plates 
XL  and  XII. ,  Part  I.,  the  hydrograph  of  1897  is  much  less  irregular 
than  that  of  1898,  the  latter  year  exhibiting  repeated  fluctuations 
in  level  due  to  floods.  As  a  result  we  find  Amoeba  occurring  rela- 
tively (to  the  number  of  collections)  almost  twice  as  often  in  1898 
as  it  did  in  1897.  It  may  also  be  significant  that  Amceba  was  not 
found  in  November  and  December,  months  of  unusual  stability  in 
river  levels.  •  There  is,  however,  a  suggestion  in  the  data  of  distri- 


97 

bution  (see  Table  I.)  that  Amoeba  may  become  an  active  member  of 
the  plankton  during  the  warmer  seasons,  like  other  Rhizopoda,  as  a 
result,  perhaps,  of  the  formation  of  gas  or  oil  vacuoles  in  its  proto-' 
plasm.  Of  the  30  occurrences,  21  fall  between  ApriLlS  and  Octo- 
ber 17,  with  water  temperatures  of  58°  and  56°,  respectively.  Of 
these  21  occurrences  in  warm  waters  but  8  accompany  flood  inva- 
sions, while  all  of  the  9  occurrences  during  the  colder  months  are  in 
connection  with  such  disturbances.  Finally,  the  maximum  num- 
ber per  cubic  meter  (6,400)  was  found  July  21  in  clear  waters,  free 
from  the  debris  of  flood  invasion.  In  conclusion,  it  seems  probable 
that  Amoeba  in  warmer  seasons  of  the  year  (above  56°)  may  adopt 
a  limnetic  habit.  There  is,  however,  the  possibility  that  local  and 
minor  disturbances  of  the  water  due  to  current,  waves,  etc.,  are 
the  occasion  of  its  presence  in  the  plankton  in  the  absence  of  flood 
conditions.  Jennings  ('OOa)  reports  both  A.  proteus  and  A.  radiosa 
in  the  open  water  of  Lake  Erie. 

The  range  of  temperature  of  river  water  in  which  Amoeba  was 
found  was  from  32°  to  89° — the  full  extremes  observed  by  us  in 
the  river  at  Havana.  The  temperature  at  the  maximum  occur- 
rence, July  21,  1897,  was  82°.  It  is  perhaps  significant  that  14 
of  the  30  occurrences  of  Amoeba  were  between  June  21  and  Sep- 
tember 6,  the  period  of  maximum  heat,  the  river  averaging 
almost  80° — apparently  the  optimum  temperature  for  the  occur- 
rence of  Amoeba  in  the  plankton  in  this  locality.  The  relative 
numbers  of  individuals  found  in  the  various  collections  of  the  five 
years  are  too  irregular  to  suggest  any  conclusions  as  to  a  seasonal 
cycle. 

Amoeba  verrucosa  Ehrbg. — Average  number,  19.  This  species 
was  found  but  three  times  in  the  plankton,  once  each  in  May, 
August,  and  September,  occurring  but  singly,  and  in  each  case  in 
flood  waters.  It  is  apparently  a  tycholimnetic  member  of  the  plank- 
ton. The  temperature  limits  of  its  recorded  occurrence  in  the 
plankton  were  5.8°  and  82°  respectively. 

Arcella. 

This  genus  is  represented  in  the  plankton  by  four  species  and 
two  varieties  which,  like  most  of  the  Rhizopoda,  are  exceedingly 
variable,  grading  in  some  instances  into  each  other  by  occasional 


98 

individuals  which  present  intermediate  characters.  The  majority 
of  the  individuals  were  taken  in  a  living  condition,  though  many 
empty  shells  were  found.  The  conditions  of  the  examination  of  the 
plankton  and  the  opacity  of  many  of  the  shells  made  it  impossible 
to  distinguish  the  dead  shells  in  all  cases.  The  records  include  many 
dead  shells. 

Arcella  costata  Ehrbg. — Average  number,  48.  For  the  purposes 
of  this  paper  I  have  included  here  all  those  individuals  which  possess 
an  angular  or  ribbed  shell.  Leidy  ('79)  refers  such  forms  to  A. 
vulgaris.  Individuals  of  this  type  are  rare,  occurring  infrequently 
and  in  small  numbers.  It  was  recorded  but  18  times  in  the  180 
collections,  and  the  largest  number  per  cubic  meter  was  only  1,187. 
As  in  the  other  species  of  the  genus,  the  warmer  months  are  favored, 
fourteen  occurrences  falling  in  June-September  in  water  at  70°  or 
above.  The  other  four  records  are  one  each  in  April,  October, 
November,  and  December.  The  seasonal  range  of  this  form  in  the 
plankton  thus  falls  in  the  main  within  the  period  of  the  maximum 
abundance  of  A.  vulgaris,  of  which  species  it  may  be  but  a  variant. 

Arcella  discoides  Ehrbg. — Average  number,  972.  This  prevalent 
species  is  not  in  all  instances  easily  separated  from  A.  vulgaris. 
Indeed,  even  Leidy  ('79)  states  that  it  graduates  into  A.  vulgaris, 
and  that  he  views  it  as  the  variety  of  this  species  in  "which  the  shell 
presents  a  greater  proportionate  reduction  in  height  compared  with 
the  breadth."  In  the  enumeration  of  our  plankton  catches,  the 
larger,  flatter,  and  unornamented  individuals  have  been  referred  to 
this  species.  Both  the  brownish  and  the  hyaline  forms  should 
probably,  for  reasons  hereafter  given,  be  included  here,  and  they 
are  so  grouped  in  the  present  discussion.  Thus  considered,  A. 
discoides  is  the  most  abundant  member  of  the  river  plankton  be- 
longing to  this  genus,  including  two  thirds  of  all  the  individuals 
observed. 

This  species  occurred  in  almost  two  thirds  of  the  collections,  hav- 
ing been  recorded  in  115  of  the  180,  and  more  frequently  and  in 
larger  numbers  in  the  latter  half  of  the  five  years  than  it  was  in  the 
earlier  period.  This  is  in  part  explained  by  the  unusual  fluctua- 
tions of  the  river  levels  in  1898,  during  the  maximum  summer 
occurrence  of  the  species.  Like  the  other  species  of  the  genus,  A. 
discoides  has  a  period  of  maximum  occurrence  in  the  latter  part  of 
summer,  as  is  shown  in  Table  I.  Of  the  115  occurrences,  55  were  in 


99 

June-September,  in  water  at  or  above  70°,  while  in  the  remaining 
eight  months  there  were  but  60  occurrences.  This  contrast  is 
heightened  by  the  ratio  of  occurrences  to  the  total  number  of  collec- 
tions, which  in  the  period  from  June  to  September  inclusive  is  55 
to  68  and  in  the  remainder  of  the  year  only  60  to  It2.  -  The  num- 
ber per  cubic  meter  is  also  higher  during  this  warm  period,  averag- 
ing for  a  single  occurrence  1,376  to  1,028  for  one  in  the  remainder  of 
the  year.  The  average  for  the  colder  months  falls  to  850  if  the 
large  accessions  attending  the  floods  of  February  and  November  are 
omitted  in  the  totals.  The  same  causes  efficient  in  determining  the 
summer  maximum  in  other  Rhizopoda  of  the  plankton  are  doubtless 
operative  here,  and  as  in  A.  vulgaris  the  impetus  of  the  summer  in- 
crease is  carried  over  into  the  autumn,  causing  a  slight  increase  in 
numbers  as  compared  with  the  numbers  at  corresponding  temper- 
atures in  the  spring  months.  It  seems  probable  that  high  temper- 
atures favor  its  occurrence  in  the  plankton,  not,  however,  directly, 
but  because  of  greater  abundance  of  food  under  those  conditions, 
greater  metabolism,  and  the  storage  of  the  products  as  oil  or  gas 
vacuoles  which  tend  to  lower  the  specific  gravity  and  thus  to  bring 
the  animal  into  the  plankton. 

The  adventitious  occurrence  of  A.  discoides  in  the  plankton  is 
shown  by  the  fact  that  45  of  the  115  occurrences  are  with  rising 
flood  waters.  The  greater  part  of  them  lie  in  the  colder  months; 
in  fact,  nine  tenths  of  the  occurrences  between  October  and  May  are 
correlated  with  flood  movements.  For  reasons  above  given,  how- 
ever, A.  discoides  may  be  regarded  as  temporarily  adopting  a  lim- 
netic habit  during  warm  months  as  a  result  of  its  physiological 
condition ;  at  least  many  individuals  of  the  species  exhibit  this  habit 
during  the  warmer  months.  The  data  do  not  indicate  that  the 
open  water  is  at  any  time  the  center  of  distribution  of  the  species. 

There  are  no  indications  of  recurrent  pulses  in  the  species  and, 
as  might  be  expected  in  case  of  adventitious  planktonts,  but  little 
evidence  of  a  characteristic  seasonal  distribution.  There  is  some 
evidence  that  the  summer  is  the  period  of  most  active  multiplica- 
tion, and  that  an  exceedingly  transparent  and  hyaline  form  other- 
wise resembling  A.  discoides  is  the  young  of  this  species.  In  1898  . 
separate  records  were  kept  of  the  two  types  with  the  result  that  they 
were  about  equally  abundant — 24,159  and  26,387  for  the  brown 
and  hyaline  types  respectively. 

(8) 


100 

With  but  few  exceptions  the  seasonal  distribution  exhibited  by 
the  hyaline  form  was  very  similar  in  time  and  numbers  to  that  of  the 
brown  form.  Both  occurred  more  frequently  and  in  larger  numbers 
in  the  warmer  months,  and  irregularly  and  in  small  numbers  in  the 
colder  waters.  Both  entered  in  larger  numbers  with  flood  waters. 
The  differences  though  slight  are  suggestive.  The  hyaline  form  was 
less  frequent  than  the  brown  both  in  occurrences  and  numbers  dur- 
ing cold  weather,  and  summer  floods  sometimes  brought  a  rela- 
tively larger  number  of  the  hyaline  type.  These  are  conditions 
that  might  be  expected  if  the  latter  is  only  the  young  (that  is,  the 
daughter  organism  occupying  the  new  shell  after  fission  of  the  oc- 
cupant of  the  old)  of  Arcella  discoides.  In  warmer  months  food  is 
more  abundant  and,  presumably,  fission  more  frequent.  For  this 
reason  the  young  individuals  abound  at  that  time.  Owing  to  the 
difference  in  the  specific  gravity  of  the  two,  the  hyaline  type  is 
more  readily  transported  by  flood  waters.  Though  not  con- 
clusive, the  data  here  presented  seem  to  favor  the  view  that  the 
hyaline  form  is  only  a  stage  in  the  life  history  of  the  individual 
Arcella  discoides. 

The  species  A.  artocrea  Leidy  and  A.  polypora  Penard  occur  also 
in  our  waters,  but  were  included  with  A.  discoides  in  the  enumera- 
tion. Typical  representatives  of  these  species  are  not,  however, 
present  in  any  numbers 

Arcella  mitrata  Leidy  was  found  but  once — on  Aug.  1,  1895,  in 
small  numbers,  at  78.5°. 

Arcella  stellata  Perty. — Under  this  designation  are  included  only 
those  individuals  which  have  well-defined  prolongations  on  the 
margin  of  the  shell.  Only  a  single  occurrence  in  small  numbers 
(48  per  cubic  meter)  was  recorded  for  the  typical  A .  stellata  —  July 
29,  1895,  at  a  temperature  of  75.5°. 

Arcella  vulgaris  Ehrbg. — Average  number,  1,098.  This  species 
is  somewhat  more  abundant  than  A .  discoides,  but  occurred  in  fewer 
collections.  It  is  a  somewhat  common  planktont,  whose  seasonal 
distribution  exhibits  some  irregularities  attributable  in  part,  as  in 
the  case  of  other  members  of  the  genus,  to  flood  conditions.  It  was 
found  in  61  of  the  180  collections  examined,  and  in  approximately 
one  third  of  those  made  in  each  year,  excepting  in  1894,  when  it 
was  not  recorded,  and  in  1898,  in  which  year  it  was  found  in  about 


101 

half  the  collections,  the  river  levels  for  this  latter  year  being  subject 
to  more  than  the  usual  disturbance. 

Arcella  vulgaris  is  found  throughout  the  whole  year,  with  a 
marked  predominance  of  occurrences  during  the  warmer  months, 
June  to  September  inclusive,  for  during  this  period,  irr  which  a  total 
of  68  collections  were  made,  this  species  was  found  in  the  plankton 
34  times.  If  the  month  of  October  be  included,  the  ratio  is  44  oc- 
currences in  83  collections,  while  in  the  remaining  97  collections, 
from  November  to  June,  only  17  occurrences  were  recorded.  Of 
the  10  occurrences  in  October,  7  were  in  water  at  or  above  55°.  The 
season  of  frequency  in  the  plankton  thus  ranges  from  June 
through  October.  In  both  frequency  of  occurrence  and  in  numbers 
of  individuals  (see  Table  I.)  there  is  an  apparent  maximum  in 
August,  preceded  by  an  increase  in  June  and  July  and  followed  by 
a  decline  in  September  and  October.  Arcella  vulgaris  thus  seems 
to  be  a  late  summer  planktont.  The  continuance  into  October 
may  in  part  be  due  to  the  temperature  conditions  above  cited,  and 
perhaps  also  to  constant  seining  of  the  river  by  fishermen  in  the 
low- water  stages  at  that  time,  causing  repeated  disturbances  of  the 
bottom  and  shores,  where  Arcella  habitually  lives.  This  maximum 
frequency  of  Arcella  during  the  warmer  months  in  the  plankton 
is,  however,  probably  due  to  the  formation  of  gas  or  oil  vacuoles  in 
the  plasma  under  the  conditions  of  higher  temperatures.  Their 
flotation  is  thus  facilitated,  and  they  become,  in  a  way,  semi-active 
but  temporary  planktonts. 

That  floods  are  also  in  part  responsible  for  the  presence  of  Arcella 
in  the  plankton  is  evident  from  the  fact  that  32  of  the  61  occurrences 
come  with  rapidly  rising  waters,  or  shortly  after  rapid  rises,  during 
the  interval  of  rapid  decline.  The  larger  numbers  of  individuals 
also  appear  in  flood- waters,  occurrences  of  more  than  1000  per  cubic 
meter  happening  10  times  with  floods  to  only  4  in  more  stable 
conditions.  The  maximum  occurrence,  25,272  per  cubic  meter, 
came  with  the  flood  of  February,  1898,  indicating  the  presence  of 
this  species  in  large  numbers,  even  under  winter  conditions,  in  some 
local  environment  tributary  to  the  flood  plankton. 

The  average  number  per  cubic  meter  in  the  61  collections  con- 
taining Arcella  was  1,260;  and  the  maximum, 25, 272, as  above  noted. 
This  species  occurred  in  only  10  collections  in  stable  conditions  of 
the  river,  when  the  temperature  of  the  water  was  below  55°.  The 


102 

average  number  of  individuals  in  these  cases  was,  however,  only 
230  per  cubic  meter  as  against  1,443  when  the  temperature  was 
above  55°,  or,  if  below,  when  floods  prevailed.  The  seasonal  and 
numerical  distribution  of  occurrences  and  individuals  alike  point 
to  the  agency  of  floods  and  higher  temperatures  in  the  introduction 
of  Arcella  into  the  plankton  from  its  usual  habitat,  the  bottom  and 
the  shore. 

This  species  occurred  in  water  ranging  in  temperature  from  32° 
to  89°.  Being  a  bottom  form,  the  plankton  data  do  not  afford  a 
satisfactory  basis  for  determining  its  true  seasonal  distribution  and 
optimum  temperature.  The  maximum  number  found,  25,272,  was 
in  water  at  32°;  but  this  was  an  isolated  occurrence  in  a  flood,  and 
serves  only  to  illustrate  the  irregularity  of  distribution  in  the 
plankton  of  tycholimnetic  organisms. 

Centropyxis  aculeaca  Stein. — Average  number,  570.  This  species 
has  appeared  in  collections  in  every  month  of  the  year,  but  its 
sequence  is  frequently  interrupted  and  its  numbers  are  quite  irregu- 
lar. Practically  without  exception  all  the  larger  occurrences  attend 
rising  flood  waters.  It  is  evidently  adventitious  at  all  seasons  of 
the  year. 

Centropyxis  aculeata  var.  ecornis  (Ehrbg.)  Leidy. — Average 
number,  604.  In  former  years  this  species  was  less  frequent  than 
the  preceding  species.  Its  appearances  in  the  plankton  tend  to 
coincide  with  those  of  C.  aculeata  (Table  I.),  and  are  doubtless  due 
to  the  same  causes.  Thus  in  the  February  flood  of  1898  there  is  a 
pulse  of  12,636  of  C.  aculeata  and  one  of  9,477  of  var.  ecornis. 
C.  l&vigata  Penard  seems  to  be  identical  with  this  variety.  The 
data  concerning  both  C.  aculeata  and  its  variety  ecornis  are  too 
irregular  to  throw  any  light  on  the  seasonal  cycle  of  these  adventi- 
tious planktonts. 

Cochliopodium  bi  limbo  sum  (Auerbach)  Leidy. — Average  number, 
1,384.  This  species  was  found  in  the  plankton  during  1898  in 
irregular  numbers  in  27  of  the  52  collections.  The  distribution  of 
the  occurrences  affoids  indubitable  proof  of  their  close  dependence 
upon  flood  waters.  In  15  of  the  27  cases  Cochliopodium  appeared 
with  a  rising  river,  and  in  all  but  6  cases,  in  periods  of  considerable 
movement  in  river  levels  (cf.  Table  I.  with  PL  XII.,  Pt.  I.),  such 
as  the  rising  flood  of  January  and  February  and  the  repeated  minor 


103 

fluctuations  of  August  and  the  following  months.  The  year  1898 
was  one  of  unusual  irregularity  in  the  hydrograph  (Pt.  I.,  PL  XII.), 
especially  at  the  lower  stages  of  the  river,  at  which  times  this 
rhizopod  appeared  most  frequently.  Its  maximum  occurrence, 
20,898  per  cubic  meter  on  Jan.  25,  accompanied  a  rise" of  0.6  of  a 
foot  in  24  hours.  At  other  times  the  numbers  range  from  100  to 
8,000  per  cubic  meter,  their  irregularity  affording  additional  ground 
for  regarding  this  species  as  an  adventitious  planktont. 

Cochlio podium  was  present  in  water  ranging  from  32.1°  to  89°, 
the  maximum  number  observed  being  found  in  water  almost  at  the 
freezing  point,  when  the  river  was  full  of  running  ice.  That  this 
is  the  optimum  temperature  for  this  organism  is  not,  however,  to 
be  inferred,  since,  as  has  been  shown  above,  this  species  is  adventi- 
tious in  the  plankton.  Plankton  collections  do  not  afford  adequate 
data  for  determining  the  seasonal  cycle  of  the  organisms  habitually 
living  upon  the  bottom.  This  species  was  not  found,  though  careful 
search  was  made  for  it,  in  the  winter  collections  of  1899.  Its 
absence  from  the  records  of  years  previous  to  1898  may  in  part  be 
due  to  a  failure  to  observe  it  in  the  silt-polluted  collections  in  which 
it  is  most  apt  to  occur. 

Cyphoderia  margaritacea  Ehrbg. — Average  number,  198.  This 
species  has  occurred  in  every  month  but  February.  In  1898,  the 
majority  of  the  occurrences  and  three  fourths  of  the  numbers  ap- 
peared between  May  1  and  October  1  at  temperatures  above  60°.  It 
was  never  abundant  at  any  time,  though  there  is  this  indication  of 
its  increased  numbers  during  the  warmer  season.  It  is  not  an  im- 
portant element  in  our  plankton.  Apstein  ('96)  found  it  somewhat 
irregularly  in  the  plankton  of  German  lakes.  In  our  waters  it 
exhibits  no  marked  dependency  upon  floods  for  its  presence  in  the 
plankton,  though  it  is  probably  capable  of  assuming  the  limnetic 
habit  in  the  warmer  season. 

Cyphoderia  trochus  Penard  appeared  occasionally  with  the  pre- 
ceding form,  from  which  it  is  distinguished  by  its  conical  horn  on 
the  fundus  and  by  its  larger  scales. 

Difflugia. 

This  genus  is  the  most  abundant  one  of  the  Rhizopoda  in  the 
plankton  of  the  Illinois  River,  and  is  a  factor  of  quantitative 


104 

importance  in  its  economy.  It  includes  a  number  of  forms  notorious 
for  their  variability  and  for  the  difficulty  with  which  specific  dis- 
tinctions can  be  applied.  I  shall  discuss  the  species  as  they  were 
enumerated,  and  shall  correlate  my  work  with  Penard's  ('02)  recent 
elaborate  analysis  of  the  species  so  far  as  I  can  with  the  aid  of  my 
notes  in  the  absence  of  the  collections.  Opinion  as  to  the  validity 
of  the  species  is  expressly  withheld  excepting  in  those  instances  in 
which  it  is  formally  stated. 

Difflugia  acuminata  Ehrbg. — Average  number,  315.  This  spe- 
cies has  occurred  in  every  month  of  the  year  and  in  83  out  of  180 
collections.  In  1898,  two  thirds  of  the  occurrences  and  three  fourths 
of  the  individuals  were  taken  between  May  1  and  October  30,  at 
temperatures  above  70°.  In  this  year  there  are  six  recurrent  pulses 
from  June  to  November,  but  all  but  one  of  these  are  found  on  rapidly 
rising  flood  waters,  and  they  bear  no  constant  relation  to  the  pulses 
of  diatoms  previously  noted,  with  which  in  some  instances  they 
are  intercalated,  though  this  is  not  regular  or  constant.  Similar 
tendencies  to  appear  with  floods  and  in  greater  numbers  and  more 
frequently  in  summer  can  be  detected  in  records  of  other  years.  It 
was  more  than  twice  as  abundant  in  1896 — a  year  of  interrupted 
hydrograph  (Pt.  I.,  PI.  X.) — as  in  1898.  This  is  one  of  the  larger 
and  heavier  rhizopods,  and  its  occurrence  in  the  plankton  is  doubt- 
less adventitious,  due  to  floods  and  currents,  and  its  greater  numbers 
and  frequency  in  the  summer  may  result  from  its  greater  abundance 
at  that  season  in  its  natural  habitat,  the  shore  and  bottom,  and 
perhaps,  also,  from  its  lighter  specific  gravity  during  the  warmer 
season.  An  illustration  of  this  appears  on  the  rising  flood  of  June, 
1897,  when  the  maximum  number  recorded  (10,000  per  m.3)  oc- 
curred. 

The  shell  of  this  species  is  exceedingly  variable  in  size,  constitu- 
ent particles,  and  proportions.  A  number  of  forms  separated  by 
Penard  ('02)  and  others  as  distinct  species  were  grouped  under  D. 
acuminata  in  the  enumeration.  The  greater  number  of  these  belong 
to  the  type  designated  by  this  name  by  Penard  ('02).  D.  acuminata 
var.  inflata  Penard  and  the  somewhat  similar  D.  elegans  Penard  are 
not  uncommon.  D.  acuminata  var.  umbilicata  Penard,  D.  elegans 
var.  teres  Penard,  D.  curmcaulis  Penard,  D.  lance olata  Penard,  and 
D.  scalpellum  Penard  occur  also,  but  are  rare. 


105 

Difflugia  bicuspidata  Rhumbler. — Average  number,  76.  A  sep- 
arate record  was  kept  of  this  bicuspid  type  in  the  later  years  of 
our  collections.  Penard  ('02)  regards  it  as  a  synonym  of  his  D. 
elegans,  though  it  would  seem  to  be  as  worthy  of  specific  distinction 
as  many  other  variants  to  which  he  accords  this  raak_  It  varies 
greatly  in  the  relative  development  of  the  accessory  "horn," 
which  is  sometimes  but  a  mere  elevation  near  the  base  of  the  main 
horn.  Individuals  with  equal  and  symmetrical  horns  represent  the 
other  extreme.  In  a  few  cases  tricuspid  individuals  have  been  seen, 
evidencing  a  tendency  to  vary  towards  the  type  found  in  D.  varians 
Penard  and  D.  fragosa  Hempel. 

This  form  was  about  one  fourth  as  abundant  as  D.  acuminata, 
and  eight  of  the  ten  occurrences  fall  between  May  and  October,  usu- 
ally with  D.  acuminata  and  presumably  for  the  same  reasons. 

Difflugiq  constricta  Ehrbg. — Average  number,  46.  This  species 
occurs  irregularly  at  all  seasons  of  the  year  without  marked  prefer- 
ence for  the  warmer  months,  and  often,  but  not  always,  with  flood 
waters.  It  occurs  throughout  the  whole  range  of  temperatures,  and 
the  largest  number  (2,778  per  m.3)  appeared  during  the  decline  of  the 
spring  flood.  Data  are  too  infrequent  to  establish  any  seasonal 
routine. 

This  species  varies  greatly,  and  is  connected  by  an  unbroken 
series  of  variants  with  the  genus  Centropyxis.  Penard  ('02)  also 
notes  the  existence  of  this  connection,  and  states  that  after  careful 
search  he  was  unable  to  find  any  constant  distinction  wrhich  would 
suffice  for  its  separation.  In  my  enumeration  only  the  elongated 
and  smooth  individuals  were  referred  to  this  species.  The  spinose 
forms  were  referred  to  Centropyxis  aculeata,  and  tho.se  similar  in 
form  to  the  spinose  type ;  but  those  free  from  spines,  to  C.  aculeata 
var.  ecornis. 

Difflugia  corona  Wallich. — Average  number,  36.  In  1896,  when 
the  hydrograph  was  much  disturbed,  the  average  number  was  more 
than  twice  as  great.  This  superb  species  was  found  in  every  month 
of  the  year  except  December,  but  never  in  large  numbers.  Its 
large  size  (200-300  fi),  and  its  heavy  shell  militate  against  its  pres- 
ence in  the  plankton,  and  its  occurrences  are  irregular  and  its  num- 
bers few.  There  is  no  marked  preference  for  warmer  months,  and 
four  fifths  of  its  occurrences  are  in  rising  flood  waters.  It  is  plainly 


106 

an  adventitious  planktont.  The  data  are  too  irregular  to  trace  its 
seasonal  distribution. 

As  a  species  it  is  as  well  defined  as  any  in  the  genus.  It  is  not  in 
our  waters  connected  by  intermediate  forms  with  other  species.  Its 
assignment  to  D.  lobostoma  by  Schewiakoff  ('93)  is  not  in  my  opinion 
justifiable  unless  we  regard  all  forms  of  Difflugia  as  belonging  to  one 
species. 

Difflugia  fragosa  Hempel. — Average  number,  25;  in  1896  over 
100.  This  species  occurred  in  every  month  of  the  year  but  Febru- 
ary, though  three  fifths  of  the  records  and  the  majority  of  the  in- 
dividuals were  found  between  May  and  October  at  temperatures 
above  60°.  The  data  are  too  irregular  to  trace  the  seasonal  history 
of  the  organism,  but  they  suffice  to  suggest  the  agency  of  floods  at 
all  times  and  of  high  temperatures  during  the  summer,  as  factors 
in  the  occurrence  of  the  species  in  the  plankton.  The  shell  of  this 
form  is  relatively  to  that  of  other  species  rather  heavy,  and  this  fact 
combined  with  the  irregularity  of  its  occurrence  seems  to  justify 
the  conclusion  that  it  is  largely  adventitious  at  all  seasons  of  the 
year. 

The  species  exhibits  a  great  deal  of  variation  in  the  development 
of  the  central  spine — Hempel  ('99,  Fig.  1) — and  in  the  number  and 
arrangement  of  spines  in  the  accessory  circlet.  The  mammillate 
form  of  the  central  spine  figured  by  Hempel  is  not  usually  present. 
Individuals  in  which  the  central  spine  is  but  feebly  developed  seem 
to  connect  this  species  with  D.  varians,  recently  described  by 
Penard  ('02).  Otherwise,  and  in  our  waters,  the  species  is  well 
delimited. 

Difflugia  globulosa  Duj. — Average  number.  7,194;  in  1897, 
47,329,  the  larger  number  in  this  year  being  in  part  due  to  a  remark- 
able pulse  of  1,240,000  early  in  September.  This  is  the  most 
abundant  of  all  the  rhizopods  in  our  plankton,  occurring  most 
frequently  and  in  largest  numbers.  It  is  found  in  every  month  of 
the  year,  and  in  1898  appeared  in  every  collection  except  four  in  De- 
cember. With  a  few  exceptions  in  the  autumn  of  1898  (Table  I.), 
no  large  development  (exceeding  10,000  per  m.3)  has  taken  place 
earlier  than  May  or  later  than  September — that  is,  at  temperatures 
below  60°.  The  occurrences  are  most  continuous  and  the  numbers 
of  individuals  are  largest  during  the  warmer  period  between  the 
months  named.  The  largest  pulse,  that  of  1,240,000  on  September 


107 

7,  1897,  was  at  80°.     A  pulse  of  48,000  on  November  22  at  40°  gives 
evidence  of  considerable  range  in  adaptation  to  temperatures. 

In  Table  I.  the  seasonal  distribution  of  D.  globulosa  is  given  in 
full.  It  differs  from  that  of  previous  years  mainly  in  the  fact  that 
the  summer  pulses  do  not  here  have  the  amplitude  reached  in  other 
years;  for  example,  in  1896  (252,000)  and  1897  (1,240,000).  It  is 
characterized  by  considerable  irregularity  caused  by  somewhat 
abrupt  pulses  at  irregular  intervals.  A  comparison  of  these  occur- 
rences with  the  hydrographic  conditions  (Pt.  I.,  PL  XII.)  indicates 
that  in  the  colder  months  increase  in  numbers  in  the  plankton  at- 
tends flood  waters  only,  as,  for  example,  in  January,  February,  late 
October,  and  November.  In  the  summer,  pulses  may  also  come 
with  floods.  For  example,  that  of  252,000  on  May  25,  1896,  ap- 
peared on  the  upward  slope  of  the  June  rise  of  the  year,  and  that 
of  80,000  on  June  28,  1897,  came  with  the  belated  June  rise  of  that 
year.  On  the  other  hand,  some  of  the  minor  fluctuations  appear 
on  declining  floods,  and  the  maximum  one  of  our  records,  that  of 
Sept.  7,  1897,  came  in  the  midst  of  the  most  prolonged  period  of 
stable  low  water  (Pt.  I.,  PL  XI.)  found  in  the  six  years  of  our 
operations.  From  these  facts  it  is  evident  that  floods  are  efficient 
in  increasing  the  number  of  D.  globulosa  in  the  plankton,  and  that 
the  amplitude  of  the  pulses  to  which  they  contribute  is  much  greater 
in  the  warmer  months  (above  60°)  than  in  the  colder  ones — as  a 
result,  perhaps,  of  the  greater  numbers  present  in  their  normal 
habitat,  the  shores  and  bottom,  and  also  as  a  result  of  their  readier 
flotation  at  this  season.  In  so  far  as  their  presence  is  due  to  floods 
they  are  adventitious.  On  the  other  hand,  it  is  very  probable  that 
they  become  temporarily  eulimnetic  in  habit  during  the  summer 
months.  The  evidence  for  this  lies  in  their  greater  numbers  in  a 
period  which  is  predominantly  one  of  greater  stability.  Thus  in 
1898,  in  the  22  collections  between  May  1  and  October  1,  the  average 
number  present  is  9,731,  while  in  the  remaining  seven  months  of 
colder  weather  the  number  is  only  5 ,200.  Additional  evidence  arises 
from  the  fact  that  pulses  of  unusual  magnitude  have  occurred  quite 
independently  of  any  factor  such  as  flood  or  other  disturbance  which 
might  cause  their  adventitious  introduction  into  the  plankton. 
Thus  on  Sept.  7,  1897,  there  is  a  symmetrical  pulse  whose  rise  and 
decline  occupy  four  weeks,  as  shown  in  the  following  table.  The 
total  change  in  river  levels  in  this  period  of  four  weeks  (Pt.  I.,  PL 


108 


Date 

Number  per  m.3 

Turbidity 
(in  meters) 

Silt 
(in  cm.3) 

Stage  of 
river  above 
low  water 

August  24        

4,800 

.37 

.15 

1.8 

August  31  

112,000 

.33 

.19 

1.8 

September  7          

1   240,000 

.15 

.45 

1.8 

September  14  

106,000 

.33 

1.04 

2.0 

September  21  

800 

.35 

trace 

2.0 

XL)  was  only  a  fall  of  .1  and  a  rise  of  .2  of  a  foot — changes  due  to 
wind  and  the  operation  of  the  locks  in  the  dams  at  either  end  of  the 
pool.  The  estimated  percentage  of  silt  is  near  the  minimum — from 
a  trace  to  5  per  cent. — and  the  turbidity  was  no  greater  than  is 
customary  (Pt.  I.,  Table  III.)  in  our  waters  during  periods  of  abun- 
dant plankton  such  as  this  (Pt.  I.,  PI.  XI.).  Beyond  the  presence 
of  these  rhizopods  there  was  nothing  in  the  plankton  to  suggest 
that  the  bottom  had  been  stirred  up  any  more  than  usual.  No 
environmental  factor  is  apparent  to  which  we  can  attribute  this 
wave  of  Difflugia  in  the  plankton.  It  is  due,  I  believe,  to  their  own 
physiological  condition.  This  was  a  time  of  prolonged  low  water 
and  great  sewage  contamination,  and  of  remarkable  development  of 
water-bloom,  chlorophyll-bearing  flagellates,  unicellular  algae,  and 
some  diatoms, — all  elements  in  the  food  of  Difflugia.  In  the  open 
water  Difflugia  could  find  abundant  sustenance  and  thus  maintain 
itself  there.  It  is  not  strange,  then,  that  we  find  it  in  these  warm 
waters,  richly  charged  with  its  food,  assuming  for  the  time  a  eulim- 
netic  habit,  perhaps  as  a  result  of  rapid  growth  and  lighter  shells,  and 
of  increased  metabolism — with  reserve  products  which  lighten  the 
specific  gravity  and  so  facilitate  flotation. 

This  species  is  found  throughout  the  whole  range  of  temperatures,. 
There  are  indications  that  its  optimum  lies  above  60°,  and  perhaps 
near  the  maximum,  80°.  This  may,  however,  be  the  result  of  the 
effect  of  temperature  upon  the  food  supply  of  the  organism.  In  any 
case  the  plankton  data  can  not  suffice  to  follow  the  complete  seasonal 
cycle  of  an  organism  which  is  either  an  adventitious  or  but  a  tem- 
porary constituent. 


109 

The  question  of  specific  limits  and  variation  in  this  organism 
is  one  of  exceeding  difficulty,  and  I  see  no  satisfactory  solution  for 
it  until  some  one  attacks  the  problem  by  a  study  of  the  variation 
by  modern  quantitative  methods,  and  endeavors  by  breeding  under 
control  to  establish  the  limits  of  variation  within  the^iormal  range 
of  seasonal  changes  of  the  environment.  When  this  is  done,  some 
more  satisfactory  criterion  for  species  in  this  group  of  planktonts 
will  be  feasible  than  the  present  condition  affords,  in  which  slight 
differences  from  previous  descriptions  are  held  to  be  valid  for  specific 
distinctions.  Thus,  in  recent  years,  species  of  plankton  Difflugia  have 
been  described  by  Heuscher  ('85)  (D.  urceolata  var.  helvetica)  from 
Swiss  lakes ;  by  Zacharias  ('97)  (D.  hydrostatica)  from  Lake  Plon ;  by 
Garbini  ('98)  (D.  cydotellind)  from  Italian  lakes;  by  Levander  ('00) 
(D.  lobostoma  var.  limneticd)  from  Finnish  waters;  and  by  Min- 
kiewitsch  ('98)  (D.  planktonicd)  from  Russian  waters.  All  of  these 
forms  occur  in  the  Illinois  River,  and  there  are  others  equally  worthy 
of  specific  designation  in  our  plankton  as  yet  undescribed.  They 
occur  most  abundantly  at  the  times  of  the  pulses,  especially  of  those 
in  stable  conditions.  In  my  opinion  they  are  all  mere  limnetic 
varieties  of  D.  globulosa  or  D.  lobostoma,  the  form  of  the  shell  and  its 
constituent  particles  being  modified  by  the  habit  of  life  in  which 
these  individuals  of  the  seasonal  cycle  are  found.  They  occur  at 
times  of  abundant  food,  rapid  multiplication,  and  limnetic  environ- 
ment. Their  shells  are  accordingly  lighter,  more  chitinous  and 
transparent,  and  the  foreign  particles  adherent  to  them  partake  of 
the  nature  of  those  of  the  silt  in  suspension.  This,  however,  is 
merely  an  opinion  based  upon  an  examination  of  the  statistics  of 
occurrences,  and  upon  the  work  of  plankton  enumeration  in  which 
all  individuals  must  be  assigned  to  some  species.  This  is  at  least  a 
different  point  of  view  from  that  of  the  systematist,  who  may,  per- 
haps, lay  more  stress  upon  divergences  from  described  types  and 
less  upon  links  connecting  such  variants.  For  the  sake  of  genuine 
progress  in  the  science  it  would  seem  to  the  writer  extremely  desir- 
able that  more  attention  be  given  to  the  question  of  variation  and 
less  to  the  description  of  new  species  under  criteria  now  in  vogue.  It 
may  be  desirable,  indeed  necessary,  to  distinguish  such  forms  in  the 
plankton.  It  would  be  both  safe  and  conservative  to  designate 
them  as  forms,  or,  at  the  most,  as  varieties. 


110 

The  location  of  the  pulses  of  D.  globulosa  bears  no  constant  rela- 
tion to  those  of  other  organisms,  owing,  in  part,  at  least,  to  the 
irregularities  of  the  floods  upon  which  some  of  them  seem  to  depend. 
The  great  pulse  of  Sept.  7,  1897,  is  intercalated  between  two  pulses 
of  diatoms  and  other  chlorophyll-bearing  organisms,  and  some 
others  bear  a  similar  relation  to  their  food  supply,  while  some  co- 
incide with  an  increase  in  these  synthetic  organisms  (cf.  Table  I. 
and  PL  II.). 

Difflugia  globulosa  and  the  following  species  were  reported  by 
Smith  ('94)  in  the  plankton  of  Lake  St.  Clair;  by  Jennings  ('OOa)  in 
that  of  Lake  Erie;  and  were  common  in  the  plankton  of  Lake 
Michigan  (Kofoid  '95).  Difflugia  of  the  forms  included  here  under 
D.  globulosa  and  D.  lobostoma  have  been  reported  by  many  authors 
from  various  European  lakes  and  rivers,  but  in  no  reported  instance 
do  they  reach  the  numbers  or  importance  in  the  plankton  that  they 
do  in  the  Illinois.  Full  records  of  their  seasonal  distribution  may, 
however,  bring  such  importance  to  light. 

Difflugia  lobostoma  Leidy. — Average  number,  1,158.  In  the 
total  of  all  collections  it  is  about  one  fifth  as  abundant  as  D.  globu- 
losa. Like  that  species  it  occurs  throughout  the  whole  year  in 
almost  every  collection  (Table  I.),  and  the  fluctuations  in  its  occur- 
rence follow  very  closely  those  just  described  for  D.  globulosa  in  the 
direction  of  their  movement.  The  amplitude  of  the  pulses  is  less,  as 
a  rule,  and  their  culminations  and  limits  are  coincident,  or  at  least 
approximate.  Thus,  on  Sept.  7,  1897,  D.  lobostoma  attains  only 
24,000,  and  the  pulse  of  D.  globulosa  on  June  28  (80,000)  is  attended 
by  one  of  96,000  in  D.  lobostoma  in  the  next  collection,  on  July  14. 
There  are  in  this  species  also  the  same  influx  into  the  plankton 
with  floods,  and  increase  in  numbers  at  temperatures  above  60°. 
There  are  954  per  collection  per  cubic  meter  below  this  temperature 
to  1,436  during  the  warmer  months  in  1898.  There  are  also  pulses 
during  the  warmer  months,  in  stable  conditions,  coincident  with 
those  of  D.  globulosa.  Similar  causes  presumably  contribute  to 
these  results  in  both  species. 

Difflugia  lobostoma  is  also  exceedingly  variable  in  proportions,  in 
the  texture  of  the  shell  and  the  degree  of  incision,  and  in  the  num- 
ber of  lobes  about  the  mouth.  Two,  three,  and  even  four  have  been 
noted,  and  they  vary  greatly  in  depth,  in  regularity,  in  perfection 
of  their  development,  and  in  the  structural  border  which  sometimes 


Ill 

forms  their  margin.  Chitinous,  brownish,  or  more  or  less  trans- 
parent shells  are  abundant  when  pulses  occur.  Forms  which 
connect  this  species  with  D.  globulosa  have  been  observed.  In- 
cluded with  D.  lobostoma  are  forms  which  have  since  been  described 
by  Penard  ('02)  as  D.  gramen,  D.  gramen  var.  acETom,  and  D. 
lithopUtes,  though  I  have  not  found  in  the  Illinois  plankton  any  of 
the  last-named  with  the  peculiar  tipped  horns  found  by  Penard 
upon  many  individuals  of  his  species. 

Difflugia  pristis  Penard  (?). — A  small  Difflugia  was  found  occa- 
sionally in  the  filter-paper  collections  in  the  colder  months,  but 
only  from  November  to  March.  It  was  often  dark,  or  even  blackish, 
resembling  in  this  respect  Penard's  D.  pristis.  Individuals  not 
thus  darkened  approach  more  nearly  D.  fallax  Penard  and  D.  puleoc 
Penard. 

Difflugia  pyriformis  Perty. — Average  number,  368.  This  species 
occurred  in  every  month  except  January,  but  generally  in  small 
numbers  and  irregularly.  The  largest  number  taken — 12,000,  on 
May  25,  1896 — came  with  the  flood  at  that  time  (Pt.  I.,  PI.  X.),  and 
all  the  large  occurrences  of  1898  came  with  rapidly  rising  water 
(cf.  Table  I:  and  Pt.  I.,  PI.  XII.).  There  are  no  indications  of  pulses 
during  stable  conditions,  and  we  must  conclude  that  the  species  is 
purely  adventitious  in  our  plankton.  It  is  one  of  the  largest  species 
with  a  heavy  shell,  and  its  flotation  is  impeded  thereby. 

This  species  is  exceedingly  variable.  The  following  varieties  or 
variants,  given  specific  rank  by  some  writers,  have  been  noted,  and 
are  included  with  D.  pyriformis  in  the  enumeration:  D.  pyriformis 
var.  nodosa  Leidy,  D.  pyriformis  var.  daviformis  Penard,  D.  pyriformis 
var.  venusta  Penard,  and  D.  pyriformis  var.  lacustris  Penard.  A 
more  slender  and  smoothly  contoured  form  than  the  last  is  not 
uncommon. 

D.  capreolata  Penard  and  D.  bacillifera  Penard  were  also  found , 
but  are  rare. 

Difflugia  rubescens  Penard  was  taken  but  once — on  May  25,  1896. 

Difflugia  tuberculosa  Hempel  was  also  found  but  once  in  the 
planktons  enumerated,  though  Hempel  ('99)  reports  it  as  appearing 
occasionally  from  August  to  November  in  1895. 

Difflugia  urceolata  Carter  was  taken  only  in  April  and  May,  1896, 
in  small  numbers  at  temperatures  of  66°- 80°. 


112 

Dinamcsba  mirabilis  Leidy  was  found  in  the  plankton  but 
once — Apr.  12,  1898,  in  small  numbers,  at  52°. 

Euglypha  alveolata  Duj.  was  found  in  small  numbers  in  the 
plankton,  but  only  on  Nov.  1,  1898,  and  March  14,  1899,  at  tempera- 
tures of  45°  and  36°. 

Euglypha  ciliata  Ehrbg.  appeared  in  the  filter-paper  collections 
in  1897,  in  July,  August,  and  November,  in  small  numbers  at  tem- 
peratures ranging  from  80°  to  48°.  This  is  said  by  Penard  ('02)  to 
be  predominantly  a  sphagnum  species,  but  widely  distributed 
elsewhere  in  small  numbers. 

Euglypha  Icevis  Perty. — This  minute  rhizopod  was  found  in  the 
filter-paper  collection  of  Oct.  4,  1898,  at  72°. 

Nebela  collaris  Leidy  was  found  only  once — on  June  25,  1898,  at 
32°. 

Pontigulasia  incisa  Rhumbler. — This  curious  rhizopod  occurred 
in  the  plankton  in  July  and  August,  1895,  and  again  in  August  and 
September,  1897,  at  temperatures  of  75°- 85°.  Both  occurrences 
were  in  stable  conditions,  and  the  temporary  adoption  of  the  lim- 
netic habit  is  suggested  by  their  appearance  at  these  times.  Two 
other  records  in  1897 — on  March  22  and  November  9,  at  44°  and 
50° —  extend  the  seasonal  range  of  the  species.  These  occurrences 
attended  rising  water  and  were  apparently  adventitious. 

Trinema  enchelys  (Ehrbg.)  Leidy. — Average  number,  158.  This 
little  cosmopolite  rhizopod  of  the  sphagnum  fauna  was  found  but 
eight  times  in  the  plankton.  The  individuals  observed  were  all  dark- 
ened by  the  granular  food  vacuoles  to  such  a  degree  that  structural 
details  were  obscured.  It  was  noted  only  in  the  somewhat  turbu- 
lent years  of  1898  and  1899,  though  on  account  of  its  small  size  and 
the  obscurity  of  its  structure  it  may  have  been  overlooked  in  previ- 
ous collections.  The  few  occurrences  are  insufficient  to  establish 
any  seasonal  routine.  They  were  at  both  extremes  of  the  tempera- 
ture range  and  in  all  seasons  but  spring,  with  a  predominance  i'n  late 
summer  and  fall.  The  species  is  evidently  adventitious  in  the 
plankton,  as  shown  by  irregular  distribution  and  small  numbers,  and 
by  the  fact  that  its  occurrences  coincide  in  all  instances  but  one  with 
rising  water. 


113 


HELIOZOA. 

The  Heliozoa  of  the  plankton  of  the  Illinois  are  few  both  in 
number  of  species  and  of  indiyiduals.  They  apparently  play  but  a 
small  part  in  the  economy  of  the  plankton.  The  average  number 
for  1898  was  but  4,883.  Their  occurrences  are  confined  -in  ^the  main 
to  midsummer  and  early  autumn.  But  four  species  were  identified, 
though  several  others  remain  undetermined  for  lack  of  sufficient 
material,  especially  of  the  living  forms.  Apstein  ('96)  reports 
Heliozoa  in  considerable  numbers  in  German  lakes,  with  maxima 
in  July- August.  It  is  probable  that  these  delicate  forms  are  fre- 
quently crushed  in  manipulation  or  hidden  in  silt  in  our  collections. 

DISCUSSION    OF    SPECIES    OF    HELIOZOA. 

-Actinophrys  sol  Ehrbg. — Average  number,  62.  This  species 
occurred  irregularly  from  April  to  the  early  part  of  November  at 
temperatures  above  46°.  It  was  recorded  most  frequently  in  the 
latter  part  of  the  summer,  the  largest  number  (28,000)  appearing 
Sept.  7,  1897,  at  80°. 

Actinosph&rium  eichhornii  (Ehrbg.)  Stein. — Recorded  a  few  times, 
from  July  to  October,  at  maximum  temperatures  (75°- 80°),  but 
always  in  small  numbers. 

Endophrys  rotatoriorum  Przesm. — This  heliozoan  (?)  has  been 
recently  described  by  Przesmycki  ( '01)  as  parasitic,  during  a  part  of 
its  existence,  in  Philodina  and  Hydatina.  A  parasite  resembling 
this  parasitic  stage  of  Endophrys  was  observed  by  me  in  a  bdelloid 
rotifer  (Rotifer  tardus)  on  several  occasions,  but  it  was  never  abun- 
dant, nor  was  its  connection  with  any  free-swimming  condition 
noted.  The  heliozoan  affinities  of  this  organism  seem  very  ques- 
tionable. 

Nuclearia  delicatula  Cienk. — Average  number,  4,760.  This 
species  in  1898  appeared  first  on  June  21,  attained  a  pulse  of  78,400 
on  August  9  at  82°  and  another  abrupt  one  of  65,600  on  September 
27  at  73°,  and  made  its  last  appearance  October  25  at  48°.  Occur- 
rences in  previous  years  are  confined  to  midsummer.  Its  optimum 
conditions  of  temperature  obviously  lie  near  the  summer  maximum, 
and  its  lower  limits  near  50°.  Its  appearance  in  the  plankton  is 
not  traceable  to  flood  conditions,  and  it  is  apparently  eulimnetic 
in  our  waters. 


114 

Hempel  ( '99)  reports  Raphidiophrys  pallida  Ehrbg.  and  R.  elegans 
Hertwig  and  Less,  in  the  plankton  of  Quiver  Lake  adjoining  the 
river,  and  I  have  found  an  undetermined  species  of  Acanthocystis 
and  a  small  heliozoan  resembling  Nuclearia  in  the  river  plankton. 

SPOROZOA. 

Triactinomyxon  sp. — In  the  plankton  collections  of  each  year 
there  have  been  found  free  limnetic  spores  which  unquestionably 
belong  to  that  highly  aberrant  and  peculiar  group  of  organisms 
described  by  Stole  ('99)  as  Actinomyxidia  and  regarded  by  him  as 
Mesozoa,  but  later  referred  by  Mrazek  ('00)  Caullery  and  Mesnil 
('04),  and  Leger  ('04)  to  the  Myxosporidia.  The  organisms  de- 
scribed by  Stole  were  parasitic  in  fresh-wrater  oligochaetes,  and  it  is 
not  improbable  that  the  limnetic  spores  taken  in  our  plankton 
collections  are  derived  from  parasites  in  some  of  the  numerous 
aquatic  oligochaetes,  or  other  invertebrates,  found  along  the  bottom 
and  shores  of  the  stream. 

The  species  here  referred  to  Triactinomyxon  differs  in  some 
details  from  T.  ignotum  Stole.  It  was  found  in  the  course  of  the 
six  years  at  least  once  in  every  month  of  the  year,  but  most  regularly 
in  May-September,  and  rarely  and  in  small  numbers  in  the  colder 
months.  Its  transparency  and  long,  slender,  radiating,  tripod-like 
arms  give  it  a  typically  limnetic  habit. 

Actinomyxidia,  gen.  et  sp.  indet. — Clusters  of  eight,  or  less, 
cylindrical  spores  radiating  from  a  common  center  and  bearing  a 
marked  resemblance  in  structural  features  to  those  of  Triactinomyx- 
on, but  lacking  any  anchor-like  projections,  were  found  sparingly 
in  the  plankton  in  June-September. 

The  distinctively  limnetic  habit  of  these  spore  stages  in  the  life- 
history  of  these  parasites  is  unique  among  the  Sporozoa,  and  has 
not,  to  my  knowledge,  been  before  noted. 

Many  of  the  rotifers  of  the  summer  plankton,  especially  Brachi- 
onus  and  an  occasional  Asplanchna,  have  been  heavily  parasitized 
internally  by  small  sac-like  bodies,  often  pear-shaped,  with  the 
smaller  end  attached  to  the  lorica,  or  of  spherical  or  flattened  form. 
They  occur  in  such  numbers  at  times  as  to  be  a  menace  to  the 
rotifer  population.  They  are  usually  most  abundant  in  any  given 
species  at  the  time  of,  or  subsequent  to,  its  maximum  occurrence.  It 


115 

was  not  unusual  to  find  as  high  as  ten  or  fifteen  per  cent,  of  the 
individuals  parasitized,  and  a  number  of  empty  loricas  bearing  addi- 
tional testimony  to  their  destructive  agency. 

Bertram  ('92)  describes  these  structures  as  "  parasitische 
Schlauche"  in  the  body  cavity  of  rotifers,  and  Przesmycki  ('01) 
works  out  their  life  history,  and  describes  the  organisms  as  Dimoe- 
rium  hyalinum,  but  does  not  designate  their  systematic  position  or 
affinities.  There  are,  however,  marked  suggestions  of  sporozoan 
affinities  in  the  organism  found  in  the  rotifers  of  the  Illinois  plankton, 
which  seems  to  be  identical  with  that  described  by  Przesmycki  ('01). 

Obviously  it  is  difficult  to  take  a  census  of  such  internal  para- 
sites. A  record  was  kept,  however,  of  the  number  of  parasitized 
individuals  in  each  species  of  rotifer,  and  references  will  be  made  to 
these  results  in  the  discussion  of  the  hosts.  Dimcerium  appeared 
in  both  summer  and  winter  rotifers,  and  its  seasonal  distribution 
naturally  depends  upon  the  number  of  available  hosts.  It  was  in 
consequence  most  abundant  during  the  midsummer  and  autumn 
months. 

CILIATA. 

Average  number,  15,812,346,  including  filter-paper  collections. 
If  these  be  excluded  and  the  silk  catches  only  averaged,  the  number 
will  fall  to  less  than  a  tenth  of  this  sum.  The  ciliates  are  found  in 
the  plankton  of  the  Illinois  throughout  the  whole  year,  and  as'  a 
whole  they  do  not  exhibit  any  common  seasonal  predominance.  The 
analysis  of  the  distribution  of  the  individual  species  which  follows, 
exhibits  two  diverse  tendencies  which  affect  the  distribution  of  the 
totals.  These  are  the  vernal  and  autumnal  pulses  of  the  Tintinnidce, 
represented  by  Codonella  cratera  and  Tintinnidium  fluviatile,  and 
the  autumnal-winter  occurrence  of  a  large  number  of  species  during 
the  height  of  the  sewage  contamination  and  bacterial  development. 
The  dominant  species  in  this  ciliate  wave  are  Carchesium  lachmanni, 
Epistylis,  Amphileptus,  Lionotus,  Plagiopyla  nasuta,  Glaucoma 
scintillans,  Stentor  niger,  and  5.  c&ruleus.  Some  species,  as  Halteria 
grandinetta,  have  a  wider  seasonal  distribution,  and  others,  as 
Vorticella,  Trichodina,  Zoothamnium,  Pyxicola  affinis,  and  many 
others,  are  adventitious  in  the  plankton.  Still  others,  as  Rhabdo- 
styla,  Cothurniopsis  vaga,  Operculana,  and  similar  peritrichan 
parasites,  are  passive  members  of  the  plankton.  The  actively 

(9) 


116 

limnetic  ciliates  are  very  few.  As  such  we  may  include  Codonella 
cratera,  Tintinnidium  fluviatile,  and  possibly  Stentor  niger.  Car- 
chesium  lachmanni  and  Epistylis  enter  the  plankton  only  in  the 
form  of  detached  and  often  moribund  zooids,  and  thus  are  not 
typical  planktonts,  though  of  quantitative  importance  in  our  plank- 
ton in  the  colder  months.  A  large  number  of  species  not  here 
reported  occur  in  our  collections  made  elsewhere  than  in  the 
river  channel,  especially  in  places  where  the  decay  of  large  quan- 
tities of  organic  matter  is  in  progress.  This  is  not  a  condition 
normally  found  in  the  open  water  of  lakes,  though  it  may  occur 
along  their  shores,  where  vegetation  is  found,  or  in  regions  of 
sewage  contamination.  In  the  waters  of  the  Illinois,  on  the 
other  hand,  the  current,  combined  with  sewage  and  industrial 
wastes  and  the  organic  detritus  from  the  richest  of  fertile  prairies, 
provides  a  suitable  environment,  even  in  the  open  water,  for 
the  support  of  a  ciliate  fauna  of  a  magnitude  somewhat  unusual 
in  fresh- water  plankton.  This  fauna  is  present  also  in  the  back- 
waters, but  is  less  abundant  there  than  in  the  river  itself.  These 
species  occur  in  greatest  numbers  of  individuals  in  our  plankton  dur- 
ing the  winter  months  at  minimum  temperatures,  rising  in  November 
as  the  temperature  falls  below  50°,  and  declining  again  as  it  rises  to 
this  point  in  April.  As  shown  by  the  bacteriological  investigations 
of  Jordan  ('00)  and  Burrill  ('02  and  '04),  the  bacterial  pulse  attend- 
ing the  decay  of  the  sewage  and  wastes  at  Peoria  does  not  reach 
Havana  during  the  warmer  months  (see  table  on  p.  231,  Pt.  I.), 
but  when  temperatures  pass  below  50°  in  November  the  increase  in 
bacteria  is  marked.  The  decay  is  less  rapid  at  low  temperatures, 
and  the  process  is  still  going  on  when  the  water  in  the  channel 
passes  Havana  during  the  prevalence  of  low  temperatures,  and  the 
ciliates  that  thrive  in  such  an  environment  abound  in  the  plankton 
at  that  time. 

The  temperature  limits  of  these  ciliates  of  the  period  of  bacterial 
development  thus  seem  to  lie  between  50°  and  32°.  An  examination 
of  the  plankton  in  the  river  at  several  points  between  Peoria  and 
Havana  at  intervals  throughout  a  year,  will  reveal  how  far  the 
component  species  of  this  ciliate  fauna  are  governed  in  their  seasonal 
distribution  in  the  plankton  at  Havana,  respectively,  by  conditions 
of  temperature  and  by  the  state  of  sewage  contamination.  The 
work  of  Roux  ('01)  upon  the  Ciliata  about  Geneva  would  seem  to 


117 

indicate  that  many  species  of  the  fauna  of  stagnant  water  are  more 
abundant  in  that  region  during  the  winter  months.  Owing  to  the 
difference  in  food  conditions  attendant  upon  the  increase  of  sewage 
and  bacteria  during  the  colder  months  in  the  Illinois  River,  it  is 
impossible  to  determine  from  the  data  at  hand  the  relative  efficiency 
of  the  two  elements  of  temperature  and  food  in  regulating  the 
seasonal  occurrences  of  our  ciliates. 

Here,  as  elsewhere,  the  disastrous  effect  of  sudden  floods  can  be 
traced.  The  number  of  ciliates  (Table  I.)  drops  as  floods  rise,  and 
recovers  as  the  waters  fall  again.  For  this  reason  the  winter  occur- 
rences of  the  total  ciliates  are  subject  to  considerable  disturbances 
in  the  winter  floods  of  the  several  years.  The  combination  of  the 
two  methods  of  collection  and  of  the  two  groups  of  ciliates,  typical 
and  adventitious,  causes  further  irregularities  (Table  I.)  in  the  sea- 
sonal distribution  of  totals. 

In  the  Illinois  River,  for  reasons  given  above,  the  Ciliata  occupy  a 
place  in  the  economy  of  the  plankton  of  more  than  the  usual  im- 
portance. They  feed  principally  upon  bacteria,  decaying  organic 
matter,  and  the  smaller  algae,  and  are  themselves  eaten  by  the 
rotifers.  I  have  found  no  evidence  that  they  are  utilized  by  the 
Entomostraca.  They  thus  become  active  agents  in  the  reduction 
of  sewage  and  in  the  destruction  of  the  bacteria  of  decay,  in  the 
purification  of  sewage-laden  waters,  and  in  the  transfer  of  the  matter 
in  sewage  to  higher  forms  of  animal  life. 

The  ciliates  found  in  the  Illinois  include  all  the  important  species 
reported  in  the  plankton  of  fresh  water,  and  the  list  is  somewhat 
larger  than  hitherto  recorded  in  quantitative  plankton  collections 
in  river  or  lake  waters.  These  organisms  escape  readily  through  the 
silk  net  by  reason  of  their  small  size,  and  in  some  instances  the 
larger  species,  by  reason  of  their  mobility  and  flexibility,  escape 
through  the  silk  where  less  motile  organisms  of  equal  size  are  re- 
tained. By  experiment  I  have  found  that  well -shrunken  silk 
bolting  -  cloth  whose  meshes  average  about  30-45/1  will  not  retain 
Paramecium  whose  diameter  is  40-70/*.  It  may  be  that  supple- 
mentary methods  of  collection  which  will  correct  the  error  of  leakage 
will  show  that  the  Ciliata  are  of  wider  occurrence  in  the  plankton 
than  has  hitherto  been  found  to  be  the  case. 


118 

DISCUSSION    OF    SPECIES    OF    CILIATA. 

Amphileptus  spp. — Average  number,  630.  Amphileptus  is  a  well- 
defined  winter  planktont  in  the  river  at  Havana,  and  it  affords  a 
striking  instance  of  the  interdependency  of  organisms  in  the  plank- 
ton. It  feeds  upon  the  heads  of  Carchesium  lachmanni,  engulfing  the 
head  in  situ  and  encysting  during  digestion.  Such  heads,  joined  to 
the  colony  or  free  in  the  plankton,  have  been  found  in  our  waters.  Its 
seasonal  distribution  at  Havana  is  almost  identical  (Table  I.)  with 
that  of  Carchesium,  upon  which  it  feeds.  Thus  in  1897-98  Car- 
chesium was  continuously  present  in  the  plankton  from  October  26 
to  May  10,  with  a  pulse  on  December  7  of  283,800,  and  one  on 
February  8  of  197,600.  Amphileptus  appears  October  26 ;  continues, 
with  interruptions,  to  May  17;  and  has  pulses  December  7  and 
January  25,  the  latter  reaching  13,545.  In  1898-99  both  appear 
early  in  October  and  have  coincident  pulses  on  November  22  and 
January  24.  In  1895-96  the  interdependence  is  even  more  striking, 
Carchesium  reaching  a  greater  development  in  this  winter,  with  a 
pulse  of  964,600  on  November  27,  and  Amphileptus  reaching  14,469  . 
on  this  date  and  14,835  a  week  later.  Both  species  decline  during 
the  flood  which  follows,  and  rise  during  March  to  culminations,  on 
the  24th,  of  104,535  and  3,636,  respectively. 

In  1898,  Amphileptus  disappears  on  April  12  at  52°,  save  for  an 
isolated  occurrence  May  17  at  64°.  It  does  not  reappear  until 
October  18  at  52°.  In  1897,  it  reappeared  October  26  at  59°,  and 
in  1895-96  its  limits  were  45°  and  48°,  with  the  exception  of  one 
occurrence,  April  17,  at  66°.  Carchesium  occurs  irregularly  and 
sparingly  during  summer  months,  and  Amphileptus  was  not  taken 
in  the  plankton  during  that  period.  Its  occurrence  in  the  plank- 
ton is  limited  in  the  main  to  temperatures  below  50°,  but  this 
limitation  may  be  due  primarily  to  the  reduced  numbers,  at  higher 
temperatures,  of  the  organism  upon  which  it  feeds.  It  appears 
during  the  period  of  greatest  sewage-contamination  and  bacterial 
development  in  the  river  at  Havana.  Roux  ('01)  finds  Amphilep- 
tus most  abundant  in  stagnant  waters  about  Geneva  in  the  winter 
months. 

Aspidisca  costata  (Duj.)  Stein. — Found  in  the  plankton  but  once 
—Jan.  11,  1898,  at  32°. 

Bursaria  truncatella  O.  F.  Mull. — Average  number,  23.  This 
large  ciliate  was  found  in  the  plankton  at  irregular  intervals  and  in 


119 

small  numbers.  It  was  found  six  times  in  March;  twice  in  January 
and  April;  and  once  in  February,  July,  and  November.  Its  ap- 
pearance in  the  plankton  is  thus  predominantly  in  winter  months 
and  at  temperatures  below  45°,  though  it  occurs  in  the  extremes 
of  temperature  conditions. 

Carchesium  lachmanni  S.  Kent. — Average  number,  26,546.  This 
is  normally  an  attached  species,  and  its  appearance  in  the  plankton 
is  due  to  the  detachment  of  the  heads.  Small  fragments  of  colonies 
are  also  found,  but  the  greater  number  are  isolated  heads.  The 
detachment  seems  to  be  a  physiological  process  of  the  organism  and 
not  merely  the  result  of  accidents.  It  is  thus  a  detached  and  an 
adventitious  planktont.  Many  of  the  heads  taken  in  the  plankton 
are  in  a  moribund  condition.  For  example,  in  a  pulse  of  March, 
1896,  the  following  proportions  were  recorded. 


Date 

Total 
Carchesium 
per  m.s 

Per  cent, 
normal 

Per  cent, 
moribund 

1896 
March  17  

60,420 

55 

45 

"      24  

104,535 

48 

52 

"      30  

47   571 

53 

47 

April  10  

16  688 

39 

61 

Enumerations  were  based  on  the  total  number  of  heads,  both 
normal  and  moribund.  The  colonies  are  sessile,  and  adhere  in  vast 
numbers  to  any  substratum  furnishing  a  suitable  place  for  attach- 
ment— submerged  vegetation,  brush,  sticks,  and  fishermen's  nets. 
The  latter  sometimes  become  so  clogged  with  Carchesium  and. 
floating  mats  of  Crenothrix  and  Beggiatoa  as  to  break  down  in  the 
current  of  the  river.  How  far  the  number  of  free  heads  in  the 
plankton  is  an  index  of  the  development  of  the  species  in  the  stream 
can  not  be  determined  from  the  data  at  hand. 

This  species  has  been  taken  in  the  plankton  in  every  month  of 
the  year,  but  its  occurrences  between  the  early  part  of  May  and 


120 

October  1 — that  is,  above  60° —  are  irregular  and  the  numbers  few 
(Table  I.).  It  is  thus  predominantly  a  cold-water  planktont. 
Winter  collections  in  1894-95  and  1896-97  were  too  few  to  trace 
its  seasonal  movements.  In  1896-97  it  appeared  November  5,  rose 
to  a  maximum  of  964,600  on  November  27,  and  declined  in  the 
December- January  flood  (Pt.  I.,  PI.  IX.)  almost  to  extinction,  but 
recovered  during  its  decline  to  a  minor  pulse  of  16,160  on  January 
30.  It  again  fell  off  in  numbers  during  the  floods  of  February 
(Pt.  I.,  PI.  X.),  but  rose  during  the  decline  of  March  to  a  maximum 
of  104,535  on  March  17.  Numbers  become  smaller  and  occurrences 
irregular  after  May  1. 

In  1897,  Carchesium  increased  rapidly  in  late  October  to  a  small 
pulse  of  13,200  on  November  2,  with  a  decline  in  the  following  fort- 
night, and  a  pulse  culminating  December  7  at  283,800,  with  subse- 
quent decline.  The  fluctuations  during  1898  may  be  followed  in 
Table  I.  The  numbers  increase  during  the  slowly  rising  flood  of 
January  to  a  maximum  of  197,600  on  February  8  at  32°,  and  decline 
again  during  the  more  rapid  rise  (Pt.  I.,  PI.  XII.)  of  the  next  three 
weeks.  Stable  conditions  in  early  March  bring  about  a  pulse  of 
89,600  on  March  15,  and  numbers  decline  again  to  2,400  as  the  flood 
passes  its  maximum  in  the  early  part  of  April.  As  the  levels  fall 
another  pulse  of  99,200  appears  April  26,  from  which  a  descent  to 
minimum  numbers — which  prevail  during  the  summer— takes  place 
within  a  fortnight.  The  floods,  especially  sudden  ones,  seem  thus 
to  interfere  with  the  appearance  of  Carchesium  in  the  plankton, 
while  gradual  rises,  as  that  of  November,  1898,  are  not  so  detri- 
mental. 

The  table  of  bacterial  occurrences  (Jordan,  '00)  in  the  Illinois  at 
Havana  and  Pekin  given  on  p.  231,  Part  I.,  indicates  that  the  bac- 
terial development  consequent  upon  the  sewage  and  industrial 
wastes  of  Peoria  extends  down  the  river  to  Havana  during  the 
colder  months  of  the  year.  The  occurrence  of  Carchesium  in  the 
plankton  is  thus  coincident  with  that  of  greatest  sewage  pollution 
and  bacterial  development  at  Havana.  Carchesium  is  much  more 
abundant  in  the  channel  of  the  river,  where  sewage  pollution  is 
greatest,  than  it  is  in  the  adjacent  backwaters.  It  seems  probable 
that  the  bacteria  either  directly  or  indirectly  contribute  towards  its 
development,  constituting,  it  may  be,  an  important  element  in  its  food. 
Flood  waters,  which  dilute  the  sewage  (cf .  hydrograph  and  chlorine 


121 

in  PL  XLV.  of  Part  I.)  might  for  this  reason  tend  to  interfere  with 
the  development  of  Carchesium,  and  thus  cut  off  the  source  from 
which  the  plankton  individuals  arise.  I  am  not  able,  however, 
to  trace  any  close  correlation  between  the  fluctuations  of  the  chem- 
ical matters  indicative  of  sewage  and  sewage  decay  and  those  of 
Carchesium.  In  the  stable  hydrographic  conditions  of  1897  we  find 
a  symmetrical  pulse  of  considerable  dimensions  rising  from  2,200 
on  November  9  to  283,800  on  December  7,  and  declining  to  26,500 
on  January  11,  1898.  Stable  low  water  with  an  ice  blockade 
(Pt.  I.,  PI.  XI.  and  XII.)  characterize  this  season.  ~Nr>  explanation 
for  the  fluctuation  is  suggested  in  the  physical  environment.  The 
chemical  condition  of  the  water,  was,  however,  greatly  disturbed 
(Pt.  I.,  PL  XLIV.).  The  fivefold  increase  in  free  ammonia  is  indic- 
ative of  approaching  stagnation  under  the  ice,  and  the  threefold 
increase  in  chlorine  marks  the  sewage  concentration.  Approaching 
stagnation  might  have  caused  the  decline  of  Carchesium,  or  it  may  be 
a  specific  reproductive  cycle  of  the  organism  which  combines  with 
the  external  factors  of  the  environment  to  produce  such  a  wave  of 
occurrence. 

Chilodon  cucullulus  Ehrbg. — Average  number,  102.  This  species 
was  found  in  the  plankton  in  January  and  February  during  the  bac- 
terial increase.  It  was  also  found  in  July.  It  escapes  through  the 
silk  net,  and  does  not  ordinarily  appear  in  plankton  collections, 
though  abundant  wherever  decay  is  active. 

Codonella  cratera  (Leidy). — Average  number,  101,024  or  452,500*. 
This  is  the  most  abundant  of  the  ciliates  in  our  plankton,  consti- 
tuting about  one  third  of  their  total  number.  It  appears  in 
every  month  of  the  year,  and  in  1898  it  was  recorded  in  every 
collection  but  one,  that  of  December  13  (Table  I.).  It  is  sub- 
ject to  great  fluctuations  in  numbers,  its  maximum  occurrences  tend- 
ing to  appear  in  April,  May,  or  June,  and  again  in  September  or 
October.  Minimum  numbers  prevail  during  the  winter,  when  many 
of  the  shells  are  empty,  and  the  midsummer  interval  is  subject  to 
pulses  of  varying  amplitude.  Spring  pulses  were  detected  as  follows : 
in  1895,  on  April  29  (16,324)  at  64°;  in  1896,  on  April  24  (562,152)  at 
72°;inl897,onApril27(470,000)at60°;andinl898,onMay3(736,000) 
at  60°.  These  vernal  pulses  coincide  with  or  approximate  closely  to 
the  dates  of  the  spring  volumetric  pulses.  This  somewhat  remark- 
able approximation  of  dates  near  the  end  of  April  may  be  the  result, 


122 

in  part  at  least,  of  the  dates  of  collection ;  but  after  allowance  is  made 
for  this,  the  species  still  exhibits  a  seasonal  cycle  of  remarkable  regu- 
larity. The  autumnal  pulse  is  of  less  amplitude,  and  of  less  regu- 
larity in  location  as  to  time  and  temperature.  In  1894  it  appears 
September  4  (14,000)  at  78°;  in  1895,  on  September  12  (5,840)  at 
81°;  in  1896,  on  August  29  (58,800)  at  74°  or  October  14  (63,200) 
at  57°;  in  1897,  on  October  5  (204,400)  at  71°;  and  in  1898,  on 
September  27  (92,800)  at  73°. 

The  midsummer  pulses  are,  as  a  rule  (Table  I.),  of  less  amplitude 
than  the  vernal  or  autumnal  ones.  In  1896  and  1898  exceptions  to 
this  statement  appear  in  two  large  developments  which  follow  in 
each  case  upon  the  decline  of  the  June  rise.  In  1896  (Pt.  I.,  PI.  X.) 
this  pulse  (152,400)  came  June  11,  and  in  1898  (Pt.  I.,  PI.  XII.)  it 
came  (1,499,200)  June  7  at  78°  and  exceeded  in  amplitude  the  re- 
corded vernal  pulse.  In  both  cases  the  pulse  was  recorded  as  occur- 
ring at  an  interval  of  a  week  after  the  crest  of  the  June  rise  had 
passed.  The  character  and  sequence  of  these  pulses  is  well  shown  in 
Table  I. 

The  occurrence  of  Codonella  in  abundance  in  the  purer  backwaters 
and  in  the  plankton  of  our  Great  Lakes  (Kofoid,  '95)  indicates  that  it 
is  not  dependent  upon  the  sewage  bacteria  directly  for  food  for  its 
development  in  our  waters.  The  appearance  of  the  greatest  pulses 
during  a  period  of  considerable  sewage  dilution  still  further  indicates 
its  independence  of  sewage  bacteria.  A  comparison  of  the  fluctua- 
tions of  the  totals  of  the  chlorophyll-bearing  organisms  with  those  of 
Codonella  affords  some  evidence  of  a  correlation  between  the  two. 
Of  39  pulses  which  can  be  traced,  in  our  records  in  the  chlorophyll- 
bearing  organisms,  21  precede  and  13  coincide  with  those  of  Codo- 
nella, while  in  the  remaining  5  instances  the  multiplication  of  Codo- 
nella precedes  that  of  the  phytoplankton  as  a  whole.  Thus  in  the 
main  the  pulses  of  Codonella  follow,  or  coincide  with,  those  of  the 
phytoplankton.  The  evidence  of  this  sequence  may  be  followed  in 
Table  I.  by  a  comparison  of  the  records  of  Codonella  with  those  of  the 
total  phytoplankton.  The  sequence  indicates  that  the  food  of  Codo- 
nella may  be  found  in  the  phytoplankton,  and  that  these  recurrent 
periods  of  growth  have  some  connection  with  the  conditions  of  nu- 
trition. The  seasonal  cycle  of  Codonella  is  closely  followed  by  the 
other  member  of  the  family  found  in  our  plankton — Tintinnidium 
fluviatile. 


123 

Codonella  occurs  throughout  the  whole  range  of  temperatures.  The 
winter  minimum  and  the  decline  during  the  maximum  temperatures 
of  summer,  combined  with  the  presence  of  vernal  and  autumnal,  or 
late  summer,  pulses,  indicate  that  the  optimum  conditions  for  this 
organism  lie  neither  in  winter  nor  in  summer.  The  spring  pulse 
was  at  temperatures  of  60°-72°,  and  the  autumnal  one  at  a  wider 
range  of  57°-78°.  Permanent  increase  in  numbers  does  not  begin 
(Table  I.)  until  March  15  at  46°,  and  the  permanent  falling  off  is 
found  on  November  15  at  41°.  The  optimum  temperatures  in  our 
waters  thus  lie  near  60°-70°,  and  conditions  favoring  growth  are 
limited  to  a  range  of  10°-15°  upon  either  side  of  the  optimum. 

This  species  readily  escapes  through  the  silk  net  on  account  of  its 
small  size  and  its  motility,  and  such  collections  give  at  the  best  in- 
complete evidence  of  its  seasonal  distribution.  The  amplitude  of  its 
fluctuations  is  thus  reduced,  and  owing  to  the  irregularity  of  the 
error  arising  from  leakage,  the  reduction  is  not  proportionally  distrib- 
uted throughout  the  year.  Tests  made  of  the  loss  of  Codonella  by 
leakage  through  the  silk  indicated  that  but  one  was  retained  to 
twenty-four  found  in  the  filtrate .  Codonella  was  counted  in  both  the  silk 
and  filter-paper  collections,  with  the  result  that  in  1897  the  totals  for 
the  year  (omitting  one  date  on  which  the  filter  collection  contained 
an  unusually  large  number  of  Codonella)  showed  one  Codonella  in  the 
silk  to  twenty-five  in  the  filter  collection.  In  1898,  however,  the 
ratio  was  one  to  four  and  a  half.  The  error  in  the  filter  collection 
is  large,  but  data  seem  to  justify  the  conclusion  that  only  a  small 
proportion  of  the  Codonella  is  retained  within  the  silk  net.  The 
proportion  for  the  whole  period  of  collection  by  the  two  methods 
(August  3,  '97,  to  March  28,  '99)  is  one  to  seven,  if  one  date  on  which 
aberrantly  large  numbers  appear  in  the  filter  collections  be  omitted. 

This  species  is  a  typical  planktont,  and  is  apparently  the  same  as 
C.  lacustris  Entz,  by  which  name  it  is  designated  by  European  writers. 
Leidy's  name,  however,  has  priority  according  to  the  accepted  rules 
of  nomenclature.  It  is  an  exceedingly  variable  organism,  at  least 
in  the  form,  proportions,  and  size  of  the  shell,  in  the  degree  of  its  con- 
striction, and  in  the  foreign  particles  which  fill  its  matrix.  The  rings 
or  bands  which  ornament  the  orifice  vary  in  their  number,  width,  and 
relative  proportions,  and  in  the  perfection  of  their  development. 
The  intergradation  which  these  variants  exhibit  is  sufficient  to  my 
mind  to  make  their  elevation  to  specific  rank  unjustifiable. 


124 

Codonella  is  an  important  element  in  the  food  of  many  of  the  lim- 
netic rotifers,  especially  Asplanchna. 

Codonella  is  a  common  constituent  in  the  plankton  of  our  own 
Great  Lakes  (Smith,  '94;  Kofoid,  '95;  Jennings,  'OOa),  and  has 
been  reported  from  most  European  waters.  Apstein  ('96)  finds  in 
German  lakes  major  pulses  in  spring  and  autumn  and  minor  ones  in 
midsummer.  Lauterborn  ('94)  reports  Codonella  in  the  plankton  of 
the  Rhine,  and  Schorler  ('00)  in  that  of  the  Elbe,  but  neither  follows 
its  seasonal  history. 

Coleps  hirtus  Ehrbg. — Average  number,  1 3 .  This  species  occurred 
in  the  plankton  collections  irregularly  and  in  small  numbers,  princi- 
pally in  autumn  months  during  the  height  of  the  bacterial  develop- 
ment. It  escapes  through  the  silk  readily. 

Colpoda  cucullus  Ehrbg*. — Average  number,  9,615.  This  species 
appears  in  the  plankton  principally  during  the  colder  months  of 
bacterial  predominance,  from  November  to  April,  and  occasionally 
during  the  summer. 

Cotkurniopsis  vaga  (Schrk.)  Blochmann  was  found  in  both  1898 
and  1899  on  Canthocamptus.  • 

Didinium  nasutum  (O.  F.  Miill.)  Stein*. — Average  number, 
12,692.  This  species  also  is  found  in  the  plankton  during  winter 
months,  especially  in  November  and  December  during  the  bacterial 
increase.  It  was  also  found  in  midsummer. 

Epistylis  spp. — Average  number,  2,020.  The  free  heads  or  frag- 
ments of  colonies  of  one,  or  possibly  of  several,  unidentified  species  of 
Epistylis,  or  it  may  be  of  Opercularia  also,  were  associated  with  Car- 
chesium  lachmanni  in  the  plankton  during  the  colder  months,  but  in 
much  smaller  numbers  (1  to  13  in  1898).  Identification  in  most  cases 
was  impracticable,  though  in  some  instances  E.  flavicans  Ehrbg.  was 
determined,  and  it  seems  probable  that  most  of  the  winter  forms  at 
least  belong  to  this  species.  Hempel  ('99)  reports  E.  plicatilis  on 
snails,  and  various  other  aquatic  animals  have  been  found  infested 
with  colonies  of  undetermined  species  of  Epistylis. 

The  distribution  of  Epistylis  in  the  plankton  (Table  I.)  is  in  its 
limits  somewhat  like  that  of  Carchesium.  It  is  more  abundant  and 
more  continuously  present  during  the  period  from  November  to  June 
(at  temperatures  below  60°)  than  in  the  intervening  warmer  months. 
It  is  found  throughout  the  whole  range  of  temperatures.  Its  pulses 
coincide  with  those  of  Carchesium  when  they  occur,  but  they  are  not 


125 

always  found  in  Epistylis  when  they  appear  in  Carchesium.  This 
degree  of  similarity  in  the  seasonal  cycle  of  the  two  genera  is  indica- 
tive of  their  correlation  with  the  same  environmental  factors,  the 
principal  one  of  which  is  the  increase  in  bacteria  attending  the  colder 
months. 

Euplotes  char  on  (O.  F.  Mull.)  Ehrbg.  was  taken  but  once  in  the 
plankton— August  23,  1898. 

Euplotes  patella  Ehrbg*. — Average  number,  2,888.  It  was  found 
in  small  numbers  and  at  irregular  intervals  from  April  to  December 
throughout  the  full  range  of  temperatures.  It  was  most  frequently 
taken  in  the  summer. 

Glaucoma  scintillans  Ehrbg.* — Average  number,  39,615.  This 
species  was  taken  in  the  plankton  from  the  middle  of  October  till  the 
middle  of  April.  It  was  present  in  larger  numbers  and  more  contin- 
uously in  December  and  February.  It  is  thus  a  member  of  the 
plankton  during  the  time  of  bacterial  increase. 

Halteria  grandinella  O.  F.  Mull.* — Average  number,  255,769. 
The  seasonal  distribution  of  this  species  in  the  plankton  does  not 
show  the  limitation  to  the  winter  months  noted  so  frequently  in  other 
ciliates.  It  was  found  in  every  month  of  the  year  but  May,  in  largest 
numbers  in  July  and  August,  and  most  continuously  in  December  and 
January.  The  data  are  too  few  and  irregular  to  determine  any  pre- 
dominance as  to  season  or  temperature. 

Holophrya  simplex  Schew.  was  found  in  small  numbers  in  the 
filter  collections  of  December,  February,  and  March  in  the  winter  of 
1896-97  at  temperatures  from  32°  to  44°. 

Leucophrydium  putrinum  Roux. — Average  number,  525.  This 
species  was  recorded  July -September,  1898,  during  the  low-water 
period,  at  temperatures  from  89°  to  63°.  It  was  described  by  Roux 
('99)  from  stagnant  water,  but  in  our  plankton  no  conditions  of  stag- 
nation attend  its  presence,  though  sewage  contamination  is  great  and 
decaying  organic  matter  abundant. 

Lionotus  spp. — Average  number,  94.  With  Amphileptus  in  the 
winter  plankton  there  occur  a  number  of  other,  smaller,  gymnostome 
ciliates  which  in  best-preserved  specimens  resemble  Lionotus.  A  few 
occurring  in  March  and  April,  1898,  were  found  to  be  L.  fasciola 
Ehrbg. ,  and  it  is  probable  that  most  of  the  individuals  belong  to  this 
species,  though  exact  identification  is  difficult  with  plankton  mate- 
rial. The  seasonal  distribution  of  Lionotus  coincides  very  closely 


126 

with  that  of  Amphileptus.  The  species  appear  in  November  or  De- 
cember and  continue  through  March  in  temperatures  below.  50°,  but 
the  numbers  retained  by  the  silk  net  are  too  small  to  trace  their  sea- 
sonal routine.  Their  seasonal  distribution  in  the  plankton  "coincides 
with  the  period  of  greatest  access  of  sewage  and  bacterial  increase  in 
the  river  at  Havana.  Roux  ('01)  finds  this  genus  well  represented  in 
the  fauna  of  swamps,  and  most  abundant  in  October  and  March. 

Loxodes  rostrum  Ehrbg.  was  identified  but  once — March  22,  1897, 
•  at  44°. 

Nassula  ntbens  Perty  occurred  July  30,  1897,  at  84°. 

Opercularia  articulata  Goldf. — This  species  is  parasitic  upon 
aquatic  Coleoptera.  In  the  plankton  of  June  28,  1897,  eleven 
colonies  or  fragments  of  a  colony  were  found,  the  largest  with  115 
zooids. 

Opercularia  nutans  (Ehrbg.). — Average  number  of  zooids,  60. 
In  the  plankton  this  species  was  found  attached  to  Alona  afjinis  in 
January,  1898,  and  to  Cyclops  in  April  and  August. 

Opercularia  not  specifically  determined  were  found  free  in  the 
plankton  in  June  and  July ;  in  November,  attached  to  Canihocamptus ; 
in  January,  attached  to  Brachionus — and  even  to  the  eggs  of  this 
species.  An  unidentified  form  was  also  found  upon  Cyclops. 

Ophryoglena  atra  Lieberk. — Five  irregular  occurrences  of  this 
species  in  small  numbers  were  recorded  in  1899  from  January  to  the 
middle  of  March. 

Paramecium  spp. — Average  number,  41.  Paramecium  was 
found  18  times  in  the  plankton.  Two  of  these  instances  were  in  May 
and  August  at  temperatures  of  64°  and  79°,  and  the  remainder  were 
between  November  20  and  March  30  at  temperatures  below  48°. 
Most  of  the  occurrences  are  in  midwinter  at  minimum  temperatures 
under  the  ice.  P.  aurelia  (O.  F.  Mull.)  has  been  found  in  the  river 
waters  (Hempel,  '99),  but  not  all  taken  in  the  plankton  belong  to  this 
species.  Specific  determinations  are  not  easily  made  with  accuracy 
in  preserved  plankton  material.  In  our  plankton,  Paramecium  is 
present  principally  during  the  period  of  greatest  contamination  by 
sewage. 

Plagiopyla  nasuta  Stein*. — Average  number,  1,181,000  during 
the  winter  of  1898-99  from  November  29  to  March  28.  This  species 
was  not  recognized  in  the  plankton  of  previous  winters.  It  reaches 
a  pulse  of  11,520,000  on  January  3,  1899,  at  32.2°  under  the  ice. 


127 

Levander  ('94)  finds  it  in  numbers  under  the  ice  in  Finnish  waters. 
On  account  of  its  motility  and  small  size  it  readily  escapes  through 
the  silk  net. 

Pleuronema  chrysalis  (Ehrbg.)  Stein. — Average  number,  9.  Re- 
corded only  in  January,  1898,  at  minimum  temperatures. 

Prorodon  farctus  Clap,  and  Lach. — Only  a  few  scattered  occur- 
rences— from  the  last  of  September  to  the  first  of  March  at  tempera- 
tures from  73°  to  minimum.  An  unidentified  species  of  Prorodon  was 
also  found  irregularly  from  November  to  April. 

Pyxicola  affinis  S.  Kent. — Average  number,  58.  This  species  is 
usually  attached  to  aquatic  plants,  especially  to  Lemna.  It  has  been 
found  in  the  summer  plankton  from  June  to  August  during  maximum 
temperatures,  especially  in  1896,  when  recurrent  floods  brought  much 
Lemna  from  the  backwaters  into  the  river.  It  was  found  October  18 
at  52°,  attached  to  Melosira  varians. 

Rhabdostyla  spp. — Average  number,  1 10.  Peritrichan  ciliates  re- 
ferred to  this  genus  have  been  noted  on  Cyclops,  Canthocamptus, 
Oligoch&ta,  and  even  in  considerable  numbers  upon  the  body,  append- 
ages, and  eggs  of  Polyarthra  platyptera.  They  have  appeared  thus 
passively  in  the  plankton  during  winter  months  from  December  to 
March,  especially  in  1899. 

Stentor  casruleus  Ehrbg. — Average  number,  882.  This  species 
presents  a  characteristic  seasonal  distribution  in  our  plankton.  Its 
numbers  are  never  very  large,  and  its  full  cycle  can  not  always  be 
traced  in  the  records.  It  is  a  planktont  of  the  colder  season  in  our 
waters.  But  three  records — one  July  28,  1896,  at  82°,  one  August  3 
of  the  same  year  at  80°,  and  a  third,  August  15,  1894,  at  84° — lie 
outside  of  the  period  between  September  1  and  May  1.  In  1898 
(Table  I.)  the  autumn  cycle  begins  September  6  at  79°,  but  in 
both  1895  and  1897  the  species  does  not  appear  until  late  in 
November  or  in  December  at  34°  or  below.  In  years  prior  to  1898 
the  numbers  were  small  and  irregular,  but  on  January  21,  1898, 
the  maximum  number  of  28,800  was  reached  at  34°,  under  the  ice, 
during  the  slowly  rising  flood  of  that  month  (Pt.  I.,  PI.  XII.).  It 
accompanied  an  increase  in  Stentor  niger,  and  there  are  indications 
elsewhere  that  the  two  species  may  fluctuate  together.  The  high 
(Pt.  I.,  PI.  XLV.)  chlorine  (38.),  nitrites  (.175),  and  free  ammonia 
(4.6)  at  the  season  of  greatest  development  in  the  plankton  are  in- 
dicative of  conditions  approaching  stagnation.  The  appearance  of 


128 

this  species  in  stagnant  water  has  often  been  observed.  Roux  ('01) 
finds  it  especially  abundant  in  September,  October,  and  February  in 
stagnant  waters  about  Geneva. 

Stentor  niger  Ehrbg. — Average  number,  3,124.  In  our  waters 
this  species  also  is  a  winter  planktont  (Table  I.).  There  have  been 
but  four  records  of  occurrence  between  May  1  and  September  1 .  In 
1895-96  the  species  appeared  November  14  at  44°  and  reached  a 
maximum  of  68,635  December  18,  after  three  weeks  of  minimum 
temperatures  and  approaching  stagnation  under  the  ice.  Numbers 
declined  in  the  December- January  flood  (Pt.  I.,  PI.  X.),  but  rose 
again  in  March,  as  the  flood  declined,  to  39,087  on  the  24th  at  40°. 
It  disappeared  from  the  plankton  April  30  at  70°  and  did  not  re- 
appear until  November  1 7 ,  from  which  time  it  continued  until  March 
22.  In  1897-98  it  returned  September  21  at  71°,  attained  a  maxi- 
mum of  42,000  November  23  at  43°,  declined  during  December,  and 
rose  to  47,000  on  January  21  at  34°  under  the  ice,  and  in  the  con- 
ditions approaching  stagnation  described  in  connection  with  the  dis- 
cussion of  S.  c&ruleus.  A  decline  in  numbers  continued  until  April 
12  at  52°.  Favorable  conditions  for  growth  are  thus  found  in  our 
waters  between  32°  and  50°,  and  the  optimum  seems  to  lie  near  40° 
or  below. 

This  species  reaches  its  greatest  development  in  our  waters  during 
the  time  of  greatest  sewage  pollution  and  bacterial  development.  It 
is  known  as  a  bog-water  species,  and  was  found  by  Roux  ('01)  in 
stagnant  waters  about  Geneva  during  the  colder  months.  Hempel 
('99)  reports  this  species  as  5.  igneus  (?),  but  from  the  descriptions 
of  Roux  ('01)  I  am  inclined  to  consider  it  as  5.  niger  Ehrbg.  It  may 
be  that  both  species  are  included  in  our  data,  but  they  are  predomi- 
nantly of  the  niger  type.  They  include  also  individuals  of  the  black- 
ish variety  5.  igneus  var.  juliginosus  Forbes,  which,  it  would  seem 
from  Roux's  description  of  these  species,  should  be  transferred  to  5. 
niger.  The  fuliginosus  form  was  very  abundant  in  the  margins  of 
Pine  and  Round  lakes,  Michigan  (Kofoid,  '95),  during  the  summer 
in  surface  temperatures  of  61°-70°,  where  sewage  contamination  was 
but  slight. 

Stentor  polymorphus  (O.  F.  Mull.)  Ehrbg.  was  found  sparingly  in 
July  and  August  during  maximum  temperatures.  Hempel  ('99) 
reports  S.  barretti  Barrett  and  5.  roeselii  Ehrbg.  from  the  river,  but 
I  have  not  identified  them  in  the  plankton  collections. 


129 

Strombidium  viride  Stein  was  found  in  small  numbers  in  January- 
March,  1899,  at  minimum  temperatures. 

Stylonychia  mytilus  (O.  F.  Mull.)  Ehrbg.  was  found  in  the  plankton 
sparingly  from  September  to  February,  and  once  in  June. 

Tintinnidium  fluviatile  Stein. — Average  number,  22,590  or  1,640,- 
192*.  This  species  is  somewhat  sharply  limited  to  the  warmer 
months  in  its  seasonal  distribution.  In  1898  (Table  I.)  it  makes  its 
appearance  April  4  at  49°,  reaches  a  maximum  of  720,000  May  3  at 
60°,  and  has  three  decreasing  pulses ;  one  of  1 04, 000^ oa  June  14  at  80°, 
one  of  95,200  on  August  2  at  79°,  and  one  of  22,400  on  September  27 
at  73°,  and  disappears  from  the  plankton  October  18  at  52°.  The 
records  in  previous  years  are  more  irregular,  though  traces  of  vernal 
and  midsummer  pulses  can  be  found  in  the  records.  Filter-paper 
catches  indicate  that  only  one  in  eighty  of  this  species  is  retained  by 
the  silk.  They  also  locate  the  pulses  as  approximately  coincident 
with  those  of  the  silk  collections. 

Apstein  ('96)  finds  Tintinnidium  to  be  a  spring  planktont  with 
its  maximum  in  April  in  Lake  Plon,  while  Seligo  ('00)  finds  it  in  lakes 
near  Danzig  in  the  autumn,  with  a  maximum  in  September.  In  our 
own  waters  in  1896  the  autumnal  pulse  in  August-September  exceeds 
the  vernal  one. 

The  gelatinous  lorica  of  this  species  is  subject  to  great  variation  in 
its  size  and  proportions,  and  especially  in  the  region  about  the  aper- 
ture. A  somewhat  thimble-shaped  form  was  described  by  Hempel 
('96)  as  T.illinoisensis,  the  specific  distinctions  being  based  wholly  on 
the  lorica.  This  form  intergrades  with  the  typical  lorica  of  T. 
•fluviatile  Stein,  and  should  not  in  my  opinion  be  given  specific  rank. 

Trachelius  ovum  Ehrbg. — Average  number  in  1895,  847.  This 
species  did  not  occur  in  1898  but  was  rather  common  in  November- 
December,  1895,  reaching  a  maximum  of  10,695  on  December  4  at 
32.5°.  Isolated  appearances  in  small  numbers  in  December  and 
January  of  other  years  have  been  recorded.  In  our  waters  it  is  thus 
a  winter  planktont.  Stagnation  conditions  under  the  ice  were 
approaching  (Pt.  I.,  PI.  XLIII.)  when  the  pulse  of  1895  occurred  in 
the  Illinois  River.  Apstein  ('96)  found  it,  however,  in  Lake  Plon  with 
a  maximum  in  May- June,  disappearing  in  the  summer  and  returning 
again  in  November. 

Trichodina  pediculus  Ehrbg. — Average  number,  1;  in  1897,  874. 
This  species  is  normally  found  upon  Hydra,  on  the  gills  and  skin  of 


130 

amphibians,  and  on  young  fish.  It  appears  in  the  plankton  during  the 
summer  months  in  every  year  except  1898,  a  single  record  only  being 
made  in  that  year.  The  earliest  record  was  on  June  1 1 ,  and  the  latest 
on  November  3 1 .  The  whole  temperature  range  is  practically  included 
in  these  occurrences,  though  the  species  disappears  within  a  few  weeks 
after  the  temperature  falls  below  50°.  It  usually  appears  in  small 
numbers  and  irregularly,  and  no  pulses  like  those  of  typical  plank- 
tonts  can  be  traced.  A  free  life  in  the  plankton  is  apparently  not  its 
usual  habit.  Zacharias  ('00)  has  recently  called  attention  to  its 
appearance  in  the  plankton  in  German  waters. 

Vorticella  rhabdostyloides  Kell. — Average  number,  61.  This  little 
Vorticella  is  found  attached  in  small  clusters  to  Anabozna  spir  aides 
and  occasionally  to  other  members  of  the  phytoplankton.  It  is  some- 
what common  in  the  waters  of  Lake  Michigan,  but  is  rare  in  spring 
months  in  the  Illinois  River. 

Vorticella  spp. — Average  number,  7,843.  At  irregular  intervals 
from  April  to  November  isolated  individuals  and  small  clusters  at- 
tached to  bits  of  debris  in  the  silt  were  taken  in  the  plankton.  They 
were  most  abundant  at  temperatures  above  50°.  The  irregularity 
in  their  occurrences  indicates  that  they  are  adventitious  in  the  plank- 
ton. Identifications  of  plankton  material  are  impracticable  except  in 
strongly  marked  species.  Hempel  ('99)  has  found  V.  campanula 
Ehrbg.,  V.  microstoma  Ehrbg.,  and  V.  similis  Stokes  in  the  river  and 
its  adjacent  waters. 

Zoothamnium  arbuscula  Ehrbg. — A  few  colonies  were  taken  in 
August  and  September  in  1896  in  the  plankton,  probably  adventitious 
during  the  disturbed  hydrograph  of  that  year  (Pt.  I.,  PL  X.). 

The  preceding  list  of  45  species  does  not  complete  the  catalog  of 
the  ciliate  constituents  of  the  plankton,  though  it  includes  all  of  the 
species  of  quantitative  importance  during  the  years  of  our  operations. 
The  residium  of  unidentified  ciliates,  which,  excluding  the  partial 
identifications  in  the  above  list,  does  not  often  exceed  two  per  cent, 
of  the  total  individual  ciliates,  includes  principally  isolated  individ- 
uals of  species  difficult  of  identification  or  others  whose  preservation 
did  not  permit  it,  and  a  considerable  number  of  small  ciliates  and  of 
forms  ectoparasitic  upon  Entomostraca  and  other  planktonts.  Most 
of  these  organisms  are  either  adventitious  or  passive  members  of  the 
plankton,  and  further  study  of  the  littoral  region,  of  stagnating 


131 

waters,  and  of  these  parasitic  forms  will  reveal  the  great  richness  of 
the  ciliate  fauna  in  this  aquatic  environment. 

SUCTORIA. 

Average  number,  332.  This  class  is  not  quantitatively  im- 
portant in  the  plankton,  being  represented,  in  so  far  as  our  records 
go,  only  by  adventitious  or  passive  planktonts.  No  limnetic  species 
has  as  yet  been  found  in  the  Illinois.  An  examination  of  the 
littoral  region  during  the  prevalence  of  ciliates  wilt  probably  yield 
a  rich  suctorian  fauna. 

DISCUSSION    OF    SPECIES   OF    SUCTORIA. 

Acineta  linguifera  Clap,  and  Lach. — This  species  is  usually  found 
on  aquatic  Coleoptera.  A  single  occurrence  of  an  unattached  indi- 
vidual was  recorded  June  21,  1898. 

Metacineta  mystacina  Ehrbg. — Average  number,  301.  This 
species  occurred  in  the  plankton  from  March  till  October  in  1898  and 
in  the  winter  months  of  1899,  at  irregular  intervals  and  in  small 
numbers  (Table  I.).  Most  of  its  occurrences  attend  flood  invasions, 
and  it  is  evidently  adventitious.  It  is  frequently  attached  in  the 
plankton  to  minute  particles  of  debris.  This  species  varies  greatly 
in  the  size  of  the  lorica.  Sand  ('01)  gives  the  range  in  height  as 
from  3  3-7 00 -/i.  The  variation  in  proportions  has  given  rise  to  a 
number  of  descriptions  of  new  species  by  Stokes  ('88  and  '94)  and 
Maskell  ('87),  but  an  examination  of  a  series  of  individuals  such  as 
appear  in  the  plankton  shows  that  they  intergrade  so  closely  that 
specific  distinctions  can  not  be  maintained  for  the  variants.  Meta- 
cineta appears  throughout  the  whole  range  of  temperatures,  no 
seasonal  predominance  appearing  in  the  records. 

Podophrya  fixa  O.  F.  Mull. — Average  number,  12.  This  species 
is  also  adventitious  in  the  plankton.  It  was  recorded  in  March  and 
September  at  37°  and  73°.  Cysts  were  noted  January  21. 

Tokophrya  quadripartite  Clap,  and  Lach. — Average  number,  4. 
Adventitious  in  the  plankton  in  March  and  November.  Hempel 
('99)  finds  it  most  abundant  in  May  and  June,  associated  with 
Epistylis  plicatilis  and  Opercularia  irritabilis  on  crayfish,  insect 
larvae,  and  turtles. 

Tokophrya  cydopum  Clap,  and  Lach. — Found  occasionally  upon 
Cyclops  during  spring  and  summer. 

(10) 


132 

PORIFERA. 

Spongilla  spp. — Average  number  of  spicules,  772.  The  identifi- 
cation of  fresh-water  sponges  by  isolated  spicules  is  practically 
impossible,  and,  moreover,  the  sponge  fauna  of  the  Illinois  River  is 
as  yet  practically  unknown.  No  attempt,  therefore,  was  made  to 
identify  the  species  to  which  the  spicules  which  occur  in  our  plank- 
ton collections  belong.  They  belong  to  the  genus  Spongilla  in  part, 
and  were  usually  the  simple  sarcode  forms,  the  gemmules  or  their 
spicules  not  appearing  in  the  plankton.  They  occurred  in  all 
months  of  the  year,  and  were  found  in  46  per  cent,  of  the  collections. 
They  are  adventitious,  and  their  occurrence  in  the  plankton  is  there- 
fore dependent  in  part  upon  hydrographic  conditions.  Records  in 
December  and  January  are  few  (3)  and  always  occur  on  rising  floods. 
In  February  and  March,  months  of  rising  floods,  they  are  increased 
(8  and  7),  but  decline  again  in  April- June  (3,5,  and  5),  months  of 
predominantly  declining  water  and  more  stable  conditions.  In 
midsummer  and  autumn  months  (July  to  November)  they  again 
occur  more  frequently  (8  to  12),  probably  as  a  result  of  proximity 
to  the  season  of  greatest  growth  and  frequency  of  sponges  in  the 
river  and  its  backwaters.  Here  also  they  occur  most  frequently 
in  years  of  greatest  hydrographic  disturbance,  as,  for  example,  in 
1898.  The  adventitious  relation  which  they  bear  to  the  plankton 
is  also  seen  in  their  erratic  and  irregular  numbers.  The  maximum 
record  (16,000  per  m.3)  was  made  June  28,  1897,  on  the  rising  flood; 
the  next  in  size,  on  August  10  in  stable  low  water.  In  both  instances 
the  plankton  was  probably  taken  from  water  in  which  as  a  result  of 
some  local  disturbance  the  remains  of  some  disintegrating  sponge 
had  been  distributed.  Living  sponges  are  found  in  considerable 
abundance  on  submerged  brush  and  timbers  in  the  channel  and 
backwaters  during  the  summer  months,  and  feed  on  the  smaller 
organisms  of  the  plankton,  being  one  of  its  depleting  agencies. 

CCELENTERATA. 

Hydra  fusca  L. — Average  number,  39.  Hydra  occurred  in  about 
16  per  cent,  of  our  channel  collections — a  percentage  which  would 
be  considerably  increased  if  the  whole  of  each  collection  had  been 
examined  for  it,  or  if  backwater  collections  should  be  included.  With 
one  exception  the  28  occurrences  recorded,  all  fall  in  May-September 


133 

at  temperatures  rarely  below  70°.  The  earliest  record  in  channel 
waters  was  on  May  1,  1896,  at  68.75°,  and  the  latest  on  November 
15,  1897,  at  47°.  Of  the  28  records  in  channel  waters  the  months 
from  May  to  September  have,  respectively,  6,  3,  10,  7,  and  1  record, 
and  there  is  1  in  November.  Hydra  is  thus  a  late  vernal  and  a 
summer  planktont  in  our  waters. 

Observations  in  the  field  and  a  cursory  examination  of  the  col- 
lections made  in  the  backwaters  have  indicated  that  Hydra  is  often 
very  abundant  on  the  vegetation.  It  is  also  limnejtic  in  habit, 
floating  with  the  foot  attached  to  the  surface  film  and  tentacles 
widely  extended;  or,  without  attachment,  in  the  deeper  strata  of 
water.  A  similar  limnetic  habit  was  often  observed  in  the  case  of 
Hydra  in  channel  waters,  especially  on  still  warm  days  when  the 
surface  was  unruffled. 

Hydra  \vas  generally  more  abundant  in  the  plankton  in  May  or 
in  early  summer.  The  maximum  record  in  channel  waters  was 
3,200  per  m.3  on  July  21,  1897,  the  error  of  dilution  being,  however, 
large  in  this  record.  In  Quiver  Lake  on  May  8,  1896,  a  maximum 
record  of  5,335  per  m.3  was  made,  the  error  of  dilution  being  very 
small.  This  was  during  a  vernal  plankton  pulse  (8.14  cm.3  per  m.3) 
in  these  waters,  when  the  food  of  Hydra  was  present  in  considerable 
abundance. 

Hydra  viridis  L.  was  seen 'frequently  in  spring-fed  backwaters 
and  in  laboratory  aquaria,  but  was  never  recognized  in  plankton 
collections  made  in  channel  or  backwaters.  The  limnetic  habit 
noted  in  H.  fusca  was  not  observed  in  the  case  of  this  species.  . 

PLATYHELMINTHES. 
TURBELLARIA. 

Numerically  and  from  the  volumetric  standpoint  the  Turbellana 
are  not  of  great  significance  in  the  plankton  of  fresh  waters  as  a  rule. 
However,  in  some  seasons  and  under  certain  conditions  Stenostoma 
becomes  very  abundant,  as,  for  example,  in  autumn  months  in  back- 
waters, and  generally  where  decaying  vegetation  abounds.  In  the 
autumn  of  1895  the  plankton  in  the  relict  pools  of  Flag  Lake  consisted 
almost  entirely  of  Synura  uvella,  Stenostoma  leucops,  and  Entomos- 
traca. 


134 

The  average  number  in  channel  waters  is  103  per  m.3,  and,  as 
might  be  expected,  their  occurrences  are  erratic  in  seasonal  distri- 
bution and  their  numbers  are  irregular.  They  occurred  in  channel 
waters  in  eveiy  month  of  the  year  and  throughout  the  whole  seasonal 
range  in  temperatures.  The  numbers  in  1898  were  larger  and  occur- 
rences more  frequent  in  May,  during  the  run-off  of  the  spring  flood, 
and  smaller  and  more  erratic  during  the  rest  of  the  year.  In  the 
total  of  all  collections  enumerated  the  percentage  of  occurrences  was 
highest  in  June  (60  per  cent.),  July  (83  per  cent.),  August  (48  per 
cent.),  and  October  (47  per  cent.),  and  lowest  in  colder  months,  when 
it  rarely  rises  above  30  per  cent.  The  numbers  are  also  larger  in  the 
warmer  months,  a  maximum  record  of  19,250  per  m.3  on  September 
4,  1894,  following  a  slight  rise  in  river  levels  at  low  stages.  The 
adventitious  character  of  the  Turbellaria  in  channel  plankton  is  sug- 
gested by  the  erratic  data,  but  the  adaptability,  at  least  of  certain 
species,  to  the  limnetic  habit  under  certain  conditions  is  also  indi- 
cated by  the  large  numbers. 

The  identification  of  the  Turbellaria  in  plankton  collections  is  not 
feasible  in  the  course  of  the  usual  methods  of  examination  of  pre- 
served plankton.  Accordingly  no  effort  was  made  to  identify  the 
individuals  occurring  in  our  catches.  Many  of  them  were  evidently 
rhabdoccele  turbellarians,  and  of  these  probably  many  were  Stenos- 
toma  leucops.  The  genus  Vortex  was  also  represented. 

Mesostomum  ehrenbergii  O.  Schmidt  was  taken  in  small  numbers 
on  August  26,  1895,  along  the  shores  of  the  river  in  vegetation.  This 
identification  is  that  of  Dr.  W.  McM.  Woodworth  ('97). 

Stenostoma  leucops  O.  Schmidt. — Average  number,  21.  By  far  the 
greater  proportion  of  the  turbellarians  in  our  collections  probably 
belong  to  this  species.  The  statements  made  regarding  the  group  as 
a  whole  therefore  probably  apply  to  this  species. 

TREMATODA. 

Many  of  our  predaceous  fishes  and  other  aquatic  vertebrates  are 
infested  to  an  extraordinary  degree  by  flukes  parasitic  in  the  intestine 
or  other  viscera.  This,  in  conjunction  with  the  fact  that  the  fish 
markets  are  located  in  house-boats  along  the  stream  and  their  refuse 
generally  cast  directly  into  the  channel,  is  sufficient  to  account  for  the 
few  adventitious  adult  distomes  which  have  been  noted  in  our  plank- 


135 

ton  collections.  They  have  occurred  singly  in  February  and  July, 
but  were  not  identified. 

The  free-swimming  larval  stages  or  cercaria  of  unidentified  trem- 
atodes  were  also  found  singly  in  August,  September,  and  October. 

Aspidogaster  conchicola  v.  Baer,  which  occurs  abundantly  in  the 
mantle  cavity  and  pericardium  of  many  of  the  Unionidcz  (see  Kelly, 
'99),  which  form  great  beds  on  the  river  bottom,  was  taken  in  an 
immature  condition  in  the  plankton  on  June  27. 

Cotylaspis  insignis  Leidy,  likewise  a  parasite  of  the  Unionidce, 
associated  with  Aspidogaster  but  confined  principally  to  the  mantle 
chamber,  was  taken  in  the  plankton  on  February  4. 

CESTODA. 

Tetrarhynchus  sp.  was  -adventitious  in  the  plankton  on  June  27, 
and  doubtless  of  similar  origin  to  the  adult  trematodes  above  noted. 

NEMERTINI. 

Fresh-water  nemerteans  were  definitely  identified  as  such  in  the 
plankton  on  only  two  occasions,  July  23,  1894,  and  March  22,  1897. 
They  were  doubtless  adventitious — from  the  shore  or  bottom,  where 
they  are  most  abundant. 

NEMATELMINTHES. 
NEMATODA. 

The  free-living  nematode  worms  are  predominantly  shore  and 
bottom  forms,  living  in  the  midst  of  the  decaying  organic  matter  of 
the  bottom  ooze.  In  a  habitat  such  as  ours,  where  the  quantity  of 
this  decaying  matter  is  very  great,  the  nematodes  are  correspondingly 
abundant,  and,  owing  to  the  unstable  hydrographic  conditions,  they 
find  many  opportunities  of  joining  the  plankton  temporarily.  Ac- 
cordingly we  find  that  nematodes  are  met  most  frequently  and  in 
largest  numbers  in  rising  flood  waters,  when  the  bottom  deposits  of 
tributaries  and  the  main  stream  are  carried  in  channel  waters  as  silt. 
Thus,  in  the  month  of  March  nematodes  occurred  in  13  of  the  15 
collections  examined,  with  an  average  number  per  m.3  of  465,  while 
in  August  they  were  found  in  but  8  of  21  collections,  and  averaged 
only  186  per  m.3.  So,  also,  in  the  winter  flood  of  1895-96  nematodes 
were  found  in  the  plankton  almost  continuously  till  the  middle  of 


136 

April,  while  in  the  more  stable  conditions  of  the  preceding  year  they 
were  found  in  only  one  third  of  the  collections.  In  1897  most  of  the 
31  collections  examined  were  made  in  stable  conditions,  and  nema- 
todes  were  found  in  but  5  of  these,  and  4  of  these  5  were  made  in 
rising  flood  waters.  In  1898,  a- year  of  greater  hydrographic  dis- 
turbance, nematodes  occurred  in  31  of  the  52  collections,  averaging 
318  per  m.3  to  82  in  1897.  Of  the  31  occurrences  in  1898  all  but  6 
were  in  recent  flood  waters.  The  hydrographic  conditions  attending 
the  presence  of  nematodes  in  the  plankton  thus  indicate  that  they  are 
adventitious  in  the  plankton.  Further  evidence  of  this  is  to  be  found 
in  their  erratic  numbers.  Thus,  on  February  20,  1896,  none  was  re- 
corded, and  on  the  25th  their  numbers  rose  in  flood  waters  to  the 
maximum  record  for  all  of  our  collections — 18,422  per  m.3 

No  effort  was  made  to  determine  the  species  of  these  nematodes. 
A  considerable  variety  of  forms  awaits  the  labors  of  some  courageous 
systematist. 

ACANTHOCEPHALA. 

These  worms  are  found  abundantly  in  the  Catostomida  and  other 
limophagous  fishes  of  the  Illinois  River,  and  in  many  of  the  water- 
fowl which  feed  in  its  waters.  A  chance  occurrence  of  a  single 
specimen  in  the  plankton  on  August  3,  1896,  is  probably  to  be  ac- 
counted for  as  in  the  case  of  other  intestinal  parasites. 

ANNULATA. 
OLIGOCH.<ETA. 

The  representatives  of  this  order  belong  to  the  smaller  aquatic 
species — generally  littoral  or  limicolous  forms  found  especially  in 
decaying  vegetation  or  among  Lemnacea,  and  belonging  principally 
to  the  family  Naidida — and  usually  occur  in  the  plankton  in 
mutilated  condition,  since  autotomy  occurs  when  the  preservative 
is  added  to  the  plankton.  Specific  identification  of  the  fragments  is 
therefore  often  impossible  and  usually  of  questionable  certainty.  I 
am  indebted  to  Professor  Frank  Smith  for  assistance  in  such  identi- 
fications as  have  been  made.  The  following  list  (see  Smith,  '00) 
gives  the  relative  frequency  of  the  species  from  which  accessions  to 
the  plankton  are  made,  with  my  notes  on  identified  forms  in  the 
plankton. 


137 


Stylaria  lacustris  (L.).  —  Abundant.  Taken  in  the  plankton  in 
April. 

Nais  elinguis  O.  F.  Mull.  —  Abundant. 

This  species  was  identified  in  the  plankton  on  April  29,  1895, 
during  the  decline  of  the  spring  flood. 

Slavina  appendiculata  (D'Udekem)  (Nais  lurida  Timm.).  —  Fre- 
quent. 

Ophidonais  serpentina  (O.  F.  Mull.)  (Nais  serpentdna  O.  F.  Mull.). 
—Frequent. 

Dero  limosa  Leidy.  —  Abundant. 

Dero  obtusa  D'Udekem.  —  Abundant. 

This  species  was  taken  in  the  plankton  in  July  and  August,  1895, 
during  the  run-off  of  impounded  waters  from  recently  invaded  back- 
waters. (See  Pt.  I.,  PI.  IX.) 

Dero  vaga  (Leidy).  —  Abundant. 

Two  individuals  (Part  I.,  p.  297)  were  found  in  channel  waters  in 
stagnation  conditions  under  the  ice  on  February  23,  1895,  at  a  time 
when  the  plankton  was  almost  entirely  exterminated.  Under 
normal  conditions  we  have  no  evidence  that  this  species  is  more 
abundant  in  stagnant  waters. 

Dero  furcata  Oken.  —  Frequent. 

Pristina  leidyi  Smith.  —  Abundant. 

Pristina  flagellum  Leidy.  —  One  specimen. 

Ch&togaster  limncsi  v.  Baer.  —  Abundant. 

Ch&togaster  diaphanus  Gruith.  —  Abundant. 

ChcEtogaster  diastrophus  Gruith.  —  This  is  apparently  the  most 
abundant  species  of  the  order  in  the  plankton,  —  having  been  identi- 
fied in  all  months  but  January  and  May,  —  especially  at  times  when 
impounded  flood  waters  are  drained  off  from  backwaters,  as,  for 
example,  in  the  March  flood  of  1895. 

jEOLOSOMATIDjE. 

ALolosoma  hemprichii  Ehrbg.  —  Frequent. 
ALolosoma  tenebrarum  Vejdovsky.  —  Abundant. 
ALolosoma  sp.  —  Abundant. 

For  reasons  assigned  above,  the  great  majority  of  the  oligochastes 
in  the  plankton  remain  unidentified  and  are  included  in  our  records 


138 

of  total  oligochsetes.  These  records  throw  some  light  on  the  condi- 
tions controlling  the  occurrence  of  oligochastes  in  the  plankton  and 
their  seasonal  distribution. 

They  occur  in  all  months  of  the  year  and  throughout  the  whole 
seasonal  range  of  temperatures.  They  appear  in  the  plankton  most 
frequently  and  in  largest  numbers  in  disturbed  hydrographic  condi- 
tions. Thus,  of  the  31  collections  made  in  1897,  only  6  contained 
oligochastes,  and  the  average  number  per  m.3  was  only  32.  Five  of 
the  6  collections  containing  oligochaetes  were  made  during  the  run-off 
of  flood  waters  from  impounding  backwaters.  In  1898,  a  year  of 
much  disturbed  hydrograph  (Part  I.,  PL  XII.),  there  were  52  col- 
lections, in  35  of  which  oligochastes  occurred  with  an  average  number 
of  76  per  m.3  Over  50  per  cent,  of  the  non-occurrences  of  oligo- 
chastes fall  in  the  more  stable  conditions  of  January,  July- August, 
and  December.  The  seasons  of  run-off  from  impounded  backwaters 
are  in  all  years  favorable  to  the  occurrence  of  oligqchastes  in  the 
plankton.  This  is  in  sharp  contrast  with  the  nematodes,  which 
appear  with  rising  floods  and  access  of  tributary  waters.  The 
oligochastes  are  thus  largely  adventitious,  at  times  when  run-off 
from  vegetation-rich  backwaters  prevails,  and  when  Lemnacece  and 
Ceratophyllum  are  washed  into  the  channel  by  hydrographic  changes. 

ROTIFERA. 
(Plates  III.  and  IV.) 

Average  number,  592,416,  of  which  195,326,  or  33  per  cent.,  are 
eggs,  free  or  carried  externally  by  the  parent.  Records  were  kept 
of  males,  of  females,  of  females  with  eggs,  of  attached  and  free, 
summer,  winter  and  male  eggs,  and  of  parasitized  and  dead  indi- 
viduals. 

Rotifers  occur  in  every  collection  and  at  all  seasons  of  the  year. 
Numbers  are  uniformly  low  (below  75,000  per  m.3  and  often  below 
15,000)  during  minimum  temperatures  from  late  in  December  till 
early  in  March.  At  other  seasons  of  the  year  numbers  fluctuate 
greatly,  rarely  reaching  the  level  of  the  winter  minimum  except 
occasionally  at  the  depressions  between  pulses.  The  curve  of 
seasonal  occurrence  falls  into  the  form  of  recurrent  pulses  (PI.  III. 
and  IV.)  previously  noted  for  other  organisms.  Of  these  pulses  the 
vernal  one  in  April-May  is  uniformly  high,  attaining  3,954,920  per 


139 

m.3  on  April  24,  1896,  2,287,160  on  May  25,  1897,  and  the  maximum 
record  of  all  years,  5,247,800,  on  May  3,  1898.  Pulses  in  excess  of 
1,000,000  per  m.3  occur  14  times  in  our  records:  in  July,  August, 
November,  and  December  in  1895;  in  April,  1896;  in  April,  May, 
September,  and  October  in  1897;  and  in  May,  June,  August,  Sep- 
tember, and  October  in  1898.  There  is,  apparently,  in  years  or 
seasons  best  represented  in  our  records,  a  tendency  for  a  vernal 
pulse,  often  the  maximum  one  of  the  year,  to  occur  in  April-May, 
and  for  an  autumnal  pulse  of  large  amplitude  to  appear  between  the 
last  of  August  and  the  middle  of  October.  The  pulses  contiguous 
to  these  major  pulses  of  the  year  are  often  of  considerable  magnitude ; 
as,  for  example,  in  1897,  when  the  maximum  of  September  7  (5,121,- 
000)  is  followed  by  another  large  pulse  on  October  12  (2,906,400), 
and  in  1898,  when  the  vernal  pulse  of  May  3  (5,247,800)  is  followed 
by  a  June  pulse,  on  the  21st,  of  large  amplitude  (2,601,200).  The 
recurrent  character  of  the  pulses  appears  throughout  maximum  and 
minimum  periods,  and  may  be  traced  in  Plates  III.  and  IV.  In  the 
period  of  15  months  from  July,  1895,  to  October,  1896,  there  are  10 
such  pulses,  and  6  months  in  which  pulses  do  not  appear.  In  the  21 
months  from  July,  1897,  to  March,  1899,  there  are  18  pulses,  and 
3  months  in  which  they  do  not  occur.  They  often  coincide  with  or 
approximate  those  of  the  Entomostraca  (PL  III.  and  IV.)  and  of  the 
chlorophyll-bearing  organisms  (PL  I.  and  II.). 

With  the  exceptions  of  the  November-December  pulses  of  1895 
at  33°  (1,595,359  on  November  27  and  1,636,640  on  December  11) 
and  the  pulse  of  October  25  (1,048,620)  at  48°,  no  pulse  of  con- 
siderable amplitude  is  found  at  temperatures  much  below  60°  in 
channel  waters. 

In  the  discussion  which  follows,  104  forms  are  listed,  6  belonging 
to  the  Rhizota,  6  to  the  Bdelloida,  91  to  the  Ploima,  and  1  to  the 
Scirtopoda. 

RHIZOTA. 

The  Rhizofa  by  virtue  of  their  fixed  habit  are  represented  in  the 
plankton  either  by  adventitious  species,  torn  from  their  location  on 
water  plants  or  other  aquatic  substrata  by  disturbances  in  the  water, 
or  by  colonial  species  with  a  free-swimming  habit,  such  as  Conochilus. 
As  represented  by  the  latter  type  they  are  of  some  quantitative  im- 


140 

portance  in  the  plankton,  especially  of  the  backwaters.     Average 
number,  8,796. 


DISCUSSION    OF    SPECIES    OF    RHIZOTA. 


Apsilus  lentiformis  Metsch. — An  Apsilus  doubtfully  referred  to 
this  species  was  taken  December  25,  1895,  and  April  29  and  July  23, 
1896,  at  temperatures  of  41°-78°,  in  each  case  with  rising  river 
levels. 

Conochilus  dossuarius  Hud. — Average  number  of  females,  517. 
This  species  was  more  than  ten  times  as  abundant  in  the  collections 
of  1896  and  1897  as  in  1898.  Hempel  ('99)  reports  it  from  January  to 
September,  with  a  maximum  in  March.  In  the  plankton  collections 
of  1896  I  did  not  record  it  until  June  11,  at  73°.  It  reached  a  maxi- 
mum of  25,800  July  18,  and  another  of  142,800  August  15,  at  86°, 
and  disappeared  from  the  plankton  September  30  at  58°.  In  1897 
it  reappeared  May  25  at  66°  and  reached  greatest  numbers  September 
7  at  80°,  and  was  not  recorded  after  the  14th.  In  1898  (Table  I.)  it 
first  occurred  March  8,  at  37°,  and  attained  its  greatest  number,  14,400, 
on  September  27  at  73°.  In  1899  it  returned  towards  the  end  of 
January,  under  the  ice,  and  continued  till  the  cessation  of  operations 
in  March.  It  thus  occurs  in  the  Illinois  throughout  practically  the 
whole-  range  of  seasonal  and  thermal  conditions,  but  not  continu- 
ously. 

Colonies  are  of  few  individuals,  and  isolated  individuals  are  often 
found  in  the  preserved  plankton.  Females  with  1-4  eggs  were  taken, 
and  were  most  numerous  during  the  rise  of  the  pulse.  About  4  per 
cent,  of  the  females  observed,  were  carrying  eggs.  Males  were  found 
on  the  decline  of  the  pulse  of  July,  1896. 

Conochilus  unicornis  Rouss. — Average  number  of  females  8,208 ; 
eggs,  100.  Recorded  from  March  15,  at  46°,  to  July  5,  at  80°.  A 
pulse  of  8,000  on  April  26  and  one  of  392,000  on  June  7  constitute 
the  only  fluctuations.  It  was  not  found  in  1897,  and  only  sporad- 
ically, during  the  summer,  in  1896.  Females  with  1-3  eggs  attend  the 
rise  of  both  pulses  in  small  numbers.  The  colonies  of  this  species 
also  are  composed  of  but  few  individuals. 

Conochilus  volvox  Ehrbg. — Average  number  of  females,  129.  A 
few  large  colonies  were  taken  March  29  and  April  5  at  49°. 

Megalotrocha  alboflavicans  Ehrbg. — Colonies  of  this  species  are 
found  in  numbers  on  Ceratophyllum  in  the  backwaters,  and  in  1894, 


141 

when  the  vegetation  was  common  along  the  margins  of  the  stream, 
it  was  taken  in  the  plankton  occasionally. 

Megalotrocha  spinosa  Thorpe. — Isolated  individuals  of  this  un- 
usual species  were  taken  in  small  numbers  in  the  plankton  in  August, 
1896,  at  maximum  temperatures,  but  no  colonies  were  observed. 
This  is  one  of  the  largest  of  the  rotifers  in  the  plankton,  individuals 
measuring  about  1  mm.  in  length.  The  species  was  described  by 
Thorpe  ('93)  from  Chinese  waters;  was  next  reported  by  Weber  ('98) 
from  the  neighborhood  of  Geneva,  Switzerland;  and Jts_ occurrence  in 
the  Illinois  is,  I  believe,  the  third  record  of  its  appearance.  It 
affords  another  illustration  of  the  cosmopolitan  nature  of  the  fresh- 
water plankton.  In  both  Chinese  and  Swiss  waters  it  was  associated 
with  M.  semibullata  Thorpe,  also  from  Hong  Kong  and  Brisbane. 
This  latter  species  occurs  in  our  waters  also  (Hempel,  '99),  though  it 
was  not  taken  in  the  plankton  with  M.  spinosa. 

BDELLOIDA. 

Average  number,  7,807.  They  were  less  numerous  in  1897,  a 
year  of  more  stable  hydrograph,  and  fully  twice  as  abundant  in  1896, 
when  river  levels  were  much  disturbed  during  summer  months.  In 
their  seasonal  distribution,  save  for  the  increase  of  Rotifer  tardus  in 
the  winter  of  1898,  the  bdelloid  rotifers  reach  their  greater  numbers 
in  the  plankton  in  the  period  from  March  to  November.  There  is  a 
trace  of  a  vernal  pulse  in  April-May  (Table  I.),  and  some  irregular 
summer  fluctuations,  attributable  in  the  main  to  floods.  Their  tem- 
perature optimum  seems  (except  in  the  case  of  R.  tardus  above  noted) 
to  lie  above  50°.  They  are  as  a  rule  adventitious  in  the  plankton, 
owing  their  presence  in  some  cases  to  floods,  though  the  vernal  in- 
crease can  not  in  most  cases  be  attributed  directly  to  this  disturbance. 
The  species  are  difficult  to  identify  in  preserved  plankton  material, 
and  the  list  here  cataloged  is  small.  Examination  of  living  plankton 
would  considerably  extend  the  list  of  forms,. 

DISCUSSION    OF    SPECIES    OF    BDELLOIDA. 

Philodina  citrina  Ehrbg.  was  found  in  the  plankton  but  once — 
September  14,  1897. 

Philodina  megalotrocha  Ehrbg. — Average  number  of  females,  351. 
This  species  was  found  in  the  plankton  (Table  I.)  from  March  15,  at 
46°,  to  November  8,  at  45°.  The  distribution  in  previous  years  fell 


142 

within  these  limits  excepting  a  single  record  December  29,  1896,  at 
35°.  The  lower  temperature  limits  are  thus  near  45°,  and  the  num- 
bers are  all  small  below  60°.  The  occurrences  are  never  in  very  large 
numbers,  and  significant  pulses  do  not  appear— an  indication  that 
the  species  is  adventitious  in  the  plankton.  The  relative  numbers  in 
different  years  is  suggestive.  In  1896,  with  a  total  movement  in 
river  levels  of  45.7  feet,  the  average  number  per  collection  is  770;  in 

1897,  with  a  total  movement  of  44.8  feet,  the  number  is  271 ;  and  in 

1898,  with  67.2  feet,  it  is  351.     In  1896  a  much  greater  proportion  of 
the  change  in  levels  took  place  (Pt.-L,  PI.  XI.)  during  the  summer, 
when  P.  megalotrocha  is  present.     With  this  in  mind,  it  is  apparent 
that  a  disturbed  hydrograph  tends  to  increase  the  number  of  this 
species  in  the  plankton.     A  comparison  of  the  individual  occurrences 
(Table    I.)   with    contemporaneous    conditions   of   the  hydrograph 
(Pt.  I.,  PI.  XII.)  in  1898,  and  in  previous  years  also,  shows  that  most 
of  the  larger  records  were  made  in  planktons  from  a  rising  river. 
For  example,  the  largest  record  made — 8,000  on  September  27,  1898 
—is  on  the  crest  of  a  slight  rise  (Pt.  I.,  PI.  XII.).     Some,  however, 
appear  in  stable  conditions,  and  may  be  attributed  to  the  other  causes 
of  disturbance  of  the  bottom  and  littoral  fauna  which  tend  to  bring 
its  constituents  temporarily  into  the  domain  of  the  plankton. 

Rotifer  neptunius  Ehrbg.- — Average  number,  425.  This  species 
was  found  in  the  plankton  in  every  month  of  the  year  but  February, 
and  thus  throughout  the  whole  temperature  range.  Between  Novem- 
ber and  March  the  records  are  scattered  and  the  numbers  small, 
while  it  is  continuously  present  in  larger  numbers  from  March  (50°) 
till  late  in  October  (50°-60°).  The  optimum  temperatures  thus  seem 
to  lie  above  50°  in  our  waters.  The  largest  numbers  recorded  (22,224, 
April  29,  1896,  at  72°,  and  6,400,  May  17,  1898,  at  64°)  attend  the 
vernal  volumetric  pulse.  Aside  from  this  season,  well-defined  and 
symmetrical  pulses  are  rarely  traceable  in  the  small  numbers  recorded. 
Some  of  the  larger  records,  for  example  that  of  July  28,  1896  (10,200), 
attend  rapidly  rising  water,  but  dependence  generally  upon  this 
agency  for  presence  in  the  plankton  is  less  directly  evident  in  this 
species  than  in  the  preceding.  As  also  in  the  case  of  R.  tardus,  the 
average  number  (246)  in  1897,  a  year  of  more  stable  hydrograph 
(Pt.  I.,  PI.  XL),  is  greatly  exceeded  by  that  in  1896  (2,323),  when  the 
hydrographic  conditions  during  summer  were  much  disturbed  (Pt.  I., 
PI.  X.). 


143 

Rotifer  spp. — Average  number,  199.  Some  bdelloid  rotifers  un- 
identified because  of  the  state  of  their  contraction,  or  not  even 
questionably  referable  to  other  species  listed,  are  here  included.  It 
is  quite  probable  that  some  individuals  belonging  to  the  genera 
Philodina  and  Callidina  are  among  the  number.  The  occurrences 
are  irregular.  They  exhibit  a  distribution  with  respect  to  years 
similar  to  that  noted  in  the  two  species  just  discussed.  Vernal  pulses 
are  noticeable  in  1896  on  April  29  (19,446),  on  April  27,  1897  (28,800), 
and  May  3,  1898  (3,200).  Egg-bearing  females  were_noted  in  the 
winter  months  of  1899,  in  December  and  March  of  the  preceding  win- 
ter, and  in  April,  1896.  Individuals  parasitized  by  Endophrys 
rotatoriorum  Przesm.  (?)  were  noted  in  April,  1896. 

Rotifer  tardus  Ehrbg. — Average  number,  6,688.  This  is  the  most 
abundant  of  all  the  bdelloid  rotifers  in  our  plankton,  outnumbering 
all  the  others  in  1898  six  to  one.  This  was  due  to  a  sporadic  and  un- 
usual pulse  of  individuals  in  the  plankton  in  midwinter  under  the  ice 
in  1898.  Owing  to  this,  the  average  number  in  1898  exceeds  that  in 
previous  years.  If,  however,  the  large  numbers  in  January  and  Febru- 
ary, 1898,  be  reduced  to  normal  winter  proportions — no  record  in 
1896  in  this  season  exceeds  7,000 — the  average  for  the  year  falls  to 
about  3,500.  The  average  of  occurrences  in  the  plankton  for  1896, 
1897,  and  1898  would  then  be  5,201, 1,254, and  3,500,  which  approxi- 
mates somewhat  the  ratios  of  the  relative  disturbance  of  the  hydro- 
graph  in  these  years  (Pt.  I.,  PI.  X.-XIL).  The  agency  of  flood 
water  in  affecting  the  numbers  of  this  species  in  the  plankton  is  to 
some  extent  indicated  by  this  ratio.  It  is  also  apparent  on  compari- 
son of  the  seasonal  distribution  (Table  I.)  with  the  hydrograph  for 
1898  (Pt.  I.,  PI.  XII.).  The  large  numbers  of  January,  February,  and 
March  appear  in  every  case  with  rapidly  rising  water,  and  the  same 
is  true  of  the  numbers  on  August  9  (12,000)  and  September  13  (17,- 
500).  Other  disturbances  than  those  due  to  floods,  or  other  factors 
than  disturbances  in  the  water,  must  be  invoked  to  explain  such  in- 
creases as  one  to  12,800  in  April-May,  1898  (Table  I.).  This  attends 
the  vernal  volumetric  pulse  (Pt.  I.,  PI.  XII.), but  does  not  conform 
to  its  proportions.  It  appears  in  the  more  stable  conditions  of  de- 
clining flood,  and  no  adventitious  factor  is  apparent  to  account  for  its 
development  to  such  numbers  in  the  plankton.  The  winter  pulse 
was  attended  by  large  numbers  of  ovigerous  females,  but  none 
was  recorded  during  this  vernal  pulse.  A  somewhat  similar  increase 


144 

in  stable  conditions  was  found  in  March  and  April,  1896,  from  40°-72°. 
Temperatures  of  50°-70°  were  several  weeks  earlier  than  usual  this 
year,  but  the  increase  in  R.  tardus  came  at  lower  temperatures  than 
in  1898. 

As  above  stated,  this  winter  pulse,  or,  rather,  sequence  of  three  pulses 
(Table  I.),  culminating  January  25  (89,397),  February  15  (27,000), 
and  March  15  (19,200),  came  with  floods.  No  such  increases  attended 
the  somewhat  similar  hydrographic  conditions  (Pt.  I.,  PI.  XIII.)  of 
1899  nor  the  winter  flood  of  1896.  There  is  nothing  in  the  eviron- 
mental  data  to  explain  this  unusual  occurrence.  An  unusually 
large  number  of  females  with  eggs  still  attached  to  the  body  were  seen 
in  the  period  from  January  21  to  April  12.  Fifty  per  cent,  were 
ovigerous,  carrying  a  single  egg.  Numbers  of  similar  free  eggs  were 
also  noted.  Rapid  multiplication  of  the  species  at  the  time  of  these 
pulses  is  thus  suggested,  and  these  may  be  dependent  upon  favorable 
conditions  of  nutrition  of  whose  nature  no  clue  is  suggested.  The 
species  is  in  the  main  adventitious,with  insufficient  evidence  of  a  par- 
tially limnetic  habit  at  some  seasons. 

The  species  occurs  in  almost  every  one  of  the  plankton  collections, 
and  thus  throughout  the  whole  range  of  temperatures  and  environ- 
mental conditions.  The  largest  numbers  were  taken  during  minimum 
temperatures  under  the  ice;  but  large  numbers  also  appear  at  other 
seasons,  and  no  temperature  optimum  is  definitely  indicated,  though 
in  years  prior  to  1898  the  larger  numbers  and  more  regular  occur- 
rences are  to  be  found  in  the  period  from  March  to  November  at 
temperatures  above  50°. 

Rotifer  vulgaris  Schrank. — Average  number,  275.  This  species 
has  a  seasonal  distribution — though  in  smaller  numbers  and  fewer 
occurrences — which  corresponds  somewhat  closely  with  that  of  R. 
neptunius  (Table  I.).  The  same  factors  in  the  environment  are  pre- 
sumably operative  in  modifying  its  appearance  in  the  plankton. 

Hempel  ('99)  finds  R.  macrurus  Schrank,  Philodina  macrostyla 
Ehrbg.,  and  Callidina  elegans  Ehrbg.  in  the  plankton  of  Quiver  Lake, 
adjacent  to  the  river. 

PLOIMA. 

Average  number,  571,611,  including  eggs,  which  constitute  about 
30  per  cent.  These  rotifers  occur  at  all  seasons  and  are  found  in 
every  collection.  They  are  quantitatively  the  most  important  order 


145 

of  the  Rotifera.     They  include  about  97  per  cent,  of  the  individuals 
and  almost  all  of  the  limnetic  species. 

As  a  group  they  exhibit  a  seasonal  routine  which  is  a  complex  of 
the  records  of  individual  species,  and  as  such  it  reflects  to  a  remark- 
able degree  a  similarity  to  individual  records,  especially  of  the  peren- 
nial species.  In  general  the  Ploima  are  less  abundant  in  colder 
months,  that  is,  below  50°-60°,  than  in  the  warmer  ones  from  May  to 
October.  Midwinter  numbers  are  nevertheless  considerable, — 5,000 
-35,000, — and  with  the  first  rise  of  temperature  in  March  we  have,  in 
1898,  a  pulse  of  175,000  which  declines  and  again  rises  in  a  vernal 
pulse  of  April-May,  which  vies  with  an  autumnal  pulse  for  rank  as 
the  annual  maximum.  Following  the  vernal  pulse  there  comes  a  series 
of  summer  movements  which  vary  from  year  to  year.  In  1898  they 
grow  smaller  as  the  season  wanes,  rising  again  in  September.  In  1897 
the  autumnal  pulse  is  the  largest  of  the  year  and  appears  early  in 
September.  In  1895,  on  the  other  hand,  it  is  carried  into  the  last 
days  of  November.  Numbers  sink  to  the  winter  minimum  shortly 
after  the  winter  temperatures  are  reached.  In  a  general  way  the 
direction  of  movement  in  the  several  parts  of  the  seasonal  curve  of  the 
total  Ploima  is  much  like  that  of  the  individual  species  of  which  it  is 
composed.  The  differences  lie  in  the  amplitude  of  the  pulses  and  in 
slight  changes  in  the  locations  of  maxima  and  minima.  There  are,  it 
is  true,  many  exceptions  to  this  sweeping  general  statement,  but  it  is, 
nevertheless,  both  surprising  and  significant  that  the  sum  of  so  many 
complex  records  should  still  preserve  the  recognizable  outlines  of  its 
parts.  This  is  not  due  simply  to  the  dominance  of  a  few  abundant 
species,  but  is  a  combination  of  many,  as  will  be  seen  frequently  ^in 
Table  I.,  where  species  with  insignificant  numbers  still  show  in  their 
seasonal  occurrences  some  correlation  with  the  movement  of  the 
great  mass  of  the  totals.  This  similarity  points  to  some  common 
factor  in  the  environment  common  to  all  of  the  species.  It  is  to  be 
found,  I  believe,  in  the  food  relations — in  the  wax  and  wane  of  the  food 
supply.  Most,  if  not  all,  ploiman  rotifers  are  herbivorous,  or  at  least 
omnivorous,  and  find  their  food  to  a  large  extent  in  the  phytoplankton. 
I  have  already  called  attention  to  the  recurrent  pulses  of  the  chloro- 
phyll-bearing organisms.  These  primarily,  but  combined  with  other 
and  largely  changing  seasonal  factors  such  as  hydrograph  and  tem- 
perature, are  the  basis  upon  which  the  superstructure  of  the  seasonal 
changes  in  the  ploiman  plankton  are  built.  The  correlation  between 


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147 

the  seasonal  distribution  of  individual  species  and  these  recurrent 
plant  pulses  will  be  discussed  in  connection  with  the  various  species 
wherever  the  data  are  available.  For  the  present  it  will  suffice  to  call 
attention  to  such  correlation  as  exists  between  fluctuations  of  the 
phytoplankton  and  the  total  Ploima.  The  table  on  the  preceding  page 
gives  the  location  and  amplitude  of  the  maxima  of  the  ploiman  pulses, 
and  a  graphic  presentation  of  the  seasonal  curve  of  distribution  of  the 
total  Rotifera  will  be  found  in  Plates  III.  and  IV.  On  comparison  of 
the  ploiman  pulses  with  those  of  the  chlorophyll-bearing  organisms, 
graphically  presented  in  Plates  I.  and  II.,  it  will  be  found  that  15  of 
the  33  pulses  of  Ploima  contained  within  the  period  covered  by  the 
plates  coincide  in  location  with  the  plant  pulses ;  that  12  follow  at  the 
next  collection,  usually  a  week  later,  and  3  within  a  fortnight ;  while 
only  3  of  the  33  exhibit  no  such  correlation.  The  data  suggest 
strongly  the  agency  of  the  plant  pulses  in  building  up  the  Ploima, 
and  that  the  food  relations  are  fundamental  in  the  fluctuations  of 
these  planktonts. 


DISCUSSION    OF    SPECIES    OF    PLOIMA. 


Anuraza  aculeata  Ehrbg. — Average  number,  1,839.  In  1898 
this  species  has  a  very  well-defined  and  characteristic  seasonal 
distribution  (Table  I.).  It  first  appears  March  8  at  37°,  increases 
to  a  maximum  of  45,200  on  May  10  at  61°.  then  declines,  and 
disappears  June  14  at  83°.  The  curve  of  its  occurrence  in  this  year 
is  a  very  symmetrical  one.  It  reappears  on  December  27  at  32°,  and 
there  are  scattered  occurrences  through  the  winter  months  of  1899. 
Records  in  other  years  suggest  in  the  main  a  similar  distribution. 
In  1896  it  first  appeared  January  6,  rose  to  a  pulse  of  6,550  on  May  8 
at  76°,  and,  on  the  decline  of  the  June  rise,  there  was  a  second  and 
larger  pulse  of  29,600  on  June  17  at  76°.  It  reappeared  on  Decem- 
ber 29,  and  in  1897  reached  a  vernal  maximum  of  22,400  on  May  25 
at  66°,  then  disappeared,  and  was  not  again  noted  in  the  following 
winter  nor  until  March  8.  In  1894  the  last  vernal  record  was  made 
June  12,  and  on  September  4,  at  78°,  there  was  an  autumnal  pulse 
of  13,825 — a  phenomenon  not  repeated  in  subsequent  years.  •  The 
normal  course  of  its  seasonal  distribution  in  the  river  plankton  seems 
to  be  as  follows:  reappearance  in  December  when  minimum 
temperatures  have  been  reached;  slow  multiplication  during  the 
winter,  and  a  well-defined  pulse  on  the  decline  of  the  spring  flood  in 

(in 


April-May  with  the  possibility  of  a  second  on  the  June  rise;  and 
prompt  and  complete  disappearance  when  maximum  summer  tem- 
peratures are  established.  Low  water  in  the  autumn  seems  to  inter- 
fere with  an  autumnal  pulse.  In  1894  there  was  a  well-sustained 
rise  in  September  (Pt.  I.,  PL  VIII.)  and  a  pulse  of  A.  aculeata.  In 
1896,  however,  no  pulse  occurred  in  the  high  water  of  the  autumn. 
No  midwinter  occurrences  followed  the  very  low  water  of  1897.  It 
is  thus  in  channel  waters  a  vernal  planktont,  with  its  temperature 
optimum  near  70°  but  below  the  summer  maximum.  Hempel's 
statement  ('99)  that  it  is  a  "winter  species"  is  borne  out  by  its 
presence  from  December  through  the  winte'r,  but  its  numerical 
distribution  ranks  it  at  once  with  the  vernal  organisms.  Lauterborn 
('94)  finds  it  abundantly  in  winter  months  in  the  Rhine,  and  Ap- 
stein  ('96)  speaks  of  it  as  a  "Sommerform,"  absent  from  Lake  Plon 
from  November  till  March,  and  with  maxima  from  April  to  July  in 
different  bodies  of  water  where  it  continues  through  the  summer 
and  till  October,  and  then  disappears.  Summer  temperatures  in 
these  waters,  however,  are  not  recorded  by  him  above  21°  C.  (69.8° 
F.),  which  is  about  the  temperature  at  the  time  of  the  vernal  maxi- 
mum in  the.  Illinois,  and  at  least  10°  F.  below  that  of  the  summer 
maximum  in  our  waters.  Jennings  ('94,  '96,  and  '00)  records  it  as 
abundant  in  the  summer  plankton  of  Lake  Erie,  Lake  Michigan,  and 
some  inland  lakes  of  Michigan.  These  waters  also  are  somewhat 
cooler  (5°-10°  F.)  than  those  of  the  Illinois  River  in  midsummer. 
Temperature,  it  seems,  must  have  a  decided  effect  upon  the  seasonal 
distribution  of  this  organism  in  our  waters,  though  the  chemical 
conditions  and  food  supply  may  also  enter  as  factors  in  the  summer 
suppression  of  the  species. 

Females  carrying  usually  a  single  egg  appeared  in  1898  early  in 
April,  and  were  most  abundant  during  the  maximum  of  the  pulse. 
On  an  average,  less  than  a  fourth  of  the  females  were  ovigerous. 
Empty  loricae  appeared  May  10  (4,800)  and  17  (3,200)  at  the  crest 
and  decline  of  the  spring  pulse,  and  the  same  phenomenon  of  deca- 
dence was  noted  in  previous  years  during  this  period.  Outbreaks 
of  parasites  were  not  recorded  for  the  species,  and  the  decline  is  to 
be  attributed  to  cessation  of  reproduction  and  to  the  death  and 
destruction  of  the  individuals  by  the  more  usual  causes. 

This  species  is  quite 'variable,  but  no  effort  was  made  to  follow 
its  seasonal  history.  The  type  form  is  by  far  the  most  abundant. 


149 

A.  aculeata  var.  valga  Ehrbg.  was  seen  frequently.  A.  serrulata 
Ehrbg.,  regarded  by  Weber  ('98)  as  a  variety  of  A.  aculeata,  was 
recorded  Jan.  24,  1899,  and  found  by  Hempel  ('99)  in  December.  It 
seems  to  be  rare  in  our  plankton.  Forms  approaching  A.  aculeata 
var.  brevispina  Gosse  were  also  noted,  but  they,  too,  are  rare,  being 
recorded  only  in  February  and  March,  1899.  A.  aculeata  var. 
curvicornis  Ehrbg.  was  noted  April  29,  1896,  at  70°. 

Anurcea  cochlearis  Gosse. — Average  number,  69,393,  distributed 
as  follows:  A.  cochlearis  (sensu  strictu)  together  with  A.  cochlearis 
var.  macracantha  Lauterborn,  9,421;  A.  cochlearis  var.  tecta  Gosse, 
15,432;  and  forms  with  posterior  spine  of  intermediate  length 
between  cochlearis  and  tecta  which  include  A.  cochlearis  var.  stipitata 
Ehrbg.,  44,540.  Numerically  this  is  one  of  our  important  species, 
containing  over  one  ninth  of  all  the  rotifers  in  1898.  It  is  surpassed 
only  by  Brachionus  bakeri  (with  varieties  included) ,  Polyarthra,  and 
SynchcBta.  Average  number  of  eggs,  32,358. 

This  is  a  perennial  planktont,  appearing  in  every  month  of  the 
year  throughout  the  whole  range  of  temperature.  Its  entire  absence 
in  August,  1898  (Table  I.),  is  not  paralleled  in  any  other  year.  In 
1897,  for  example,  there  is  a  well-developed  pulse  of  45,600  on 
August  24.  In  1894,  1895 ,  and  1896  there  is  a  midsummer  minimum 
of  a  few  weeks'  duration  in  July,  August,  or  September,  but  it  is 
irregular  in  its  location. 

While  the  appearance  of  sexual  cycles  was  not  traced  by  the 
records  of  males  and  winter  eggs, — a  matter  of  some  difficulty  and 
uncertainty  in  preserved  plankton  material, — the  existence  of  such 
cycles  is  suggested  by  the  recurrent  pulses  of  occurrence  in  this 
species  (Table  I.).  It  is  possible  that  the  species  is  poly  cyclic  in 
our  waters.  The  pulses  in  1898  are  well  defined,  in  fact,  somewhat 
better  than  in  previous  years.  The  following  table  gives  the  num- 
bers in  the  pulses  in  the  several  years  and  the  dates  and  tempera- 
tures at  which  the  maxima  occurred. 

All  of  the  large  pulses  save  those  of  November  and  December 
and  one  at  the  close  of  October  (Oct.  25,  1898,  28,500)  lie  at  tempera- 
tures above  60°.  The  vernal  pulse  of  April-May  is  the  largest  and 
appears  between  60°  and  70°,  and  the  amplitude  diminishes  as  the 
period  of  maximum  heat  progresses,  though  in  1898  there  was  a 
recurrence  of  larger  numbers  as  temperatures  fell.  The  optimum 


150 


PULSES  OF  ANUR^EA  COCHLEARIS. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1896 

May  8 

76° 

100,870 

June  11 

73° 

95,200 

July      2 
28 

81° 
81° 

12,800 
17,600 

1898 

May  10 

62° 

1,145,600 

June  21 

77° 

372,800 

July    19 

84° 

17,200 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 

Sept.     4 
Sept.  23 

78° 
76° 

7,350 
1,521 

Aug.  21 

83° 

17,805 

Nov.  20 

44° 

1,120 

1896 
1897 
1898 

Aug.  21 
Aug.  24 

79° 
78° 

5,600 
45,600 

Sept.  16 
Oct.      5 

Sept.  27 
Oct.    25 

71° 
70° 

73° 
48° 

6,224 
4,800 

54,400 
28,500 

Dec.    29 

35° 

3,840 

Nov.  21 

40° 

10,000 

conditions  seem  thus  to  be  found  in  the  river  at  temperatures  some- 
what below  the  maximum,  between  60°  and  70°. 

The  phenomena  of  recurrent  pulses  are  distinctly  traceable  in  the 
seasonal  distribution  of  this  species,  not  only  in  1898  (Table  I)  but 
also  in  preceding  years.  The  large  May  and  June  pulses  of  1898 
appear  on  the  declines  of  the  spring  and  the  June  rise,  respectively; 
the  pulse  of  September  27  is  in  a  falling  river;  and  that  of  October 
25,  on  a  slowly  rising  flood  (Pt.  I.,  PI.  XII.).  In  1897  (Pt.  L,  PL 
XI.)  the  first  two  pulses  attend  the  spring  flood  and  June  rise  in  like 
manner,  but  the  two  subsequent  pulses  are  in  stable  low  water.  In 
1896  five  of  the  seven  pulses  lie  on  the  declines  of  the  recurrent 
floods  of  that  year  and  two  in  rising  waters  (cf.  PI.  X.  of  Pt.  I.  and 
the  table  just  given).  In  1894  and  1895  the  pulses  appear  either 
in  falling  water  or  in  the  earliest  stages  of  the  rise.  The  number  of 
pulses  on  declining  waters  is  somewhat  greater  than  the  relative 
number  of  days  of  this  condition  w7ould  lead  us  to  expect,  and  it 
seems  probable  that  optimum  conditions  for  the  appearance  of 
larger  numbers  of  Anuraza  cochlearis  are  to  be  found  in  such  hydro- 
graphic  conditions.  The  run-off  of  impounded  backwaters  is  one 
of  the  favorable  phases  during  flood  decline.  On  the  other  hand. 


151 

the  distribution  of  the  pulses  with  reference  to  the  floods  and  the 
appearance  of  pulses  during  rising  water  suggest  the  operation  of 
other  factors  than  the  one  arising  from  contribution  from  back- 
waters. 

The  pulse  must  be  dependent  to  a  large  extent  upon  food  supply 
of  the  organism,  and  a  correlation  between  its  periods  of  multiplica- 
tion and  the  pulses  of  its  food,  the  chlorophyll-bearing  organisms,  is 
to  be  expected.  A  comparison  of  the  seasonal  distribution  in  1898 
(Table  I.)  and  the  pulses  of  chlorophyll-bearing  organisms  (PL  II.) 
reveals  the  fact  that  three  of  the  A.  cockle aris  pulses  coincide  with 
those  of  the  plants  constituting  their  food,  and  the  other  three 
coincide  in  part  only,  the  remainder  of  the  chlorophyll-bearing 
groups  reaching  their  culmination  a  week  prior  to  that  of  the  rotifer. 
In  1897  the  three  pulses  of  A.  cochlearis  which  lie  in  the  common 
period  (PI.  II.)  all  culminate  a  week  (in  one  case  in  part  in  fourteen 
days)  after  the  maximum  of  the  plants  in  question.  In  1896,  three 
pulses  coincide  and  three  follow  in  the  subsequent  collection;  and 
in  1895,  two  coincide  and  two  follow.  Collections  at  daily  intervals 
would  be  necessary  to  follow  the  correlation  more  accurately.  It  is 
probable  from  these  juxtapositions  and  sequences  in  the  A. 
cochlearis-algse  pulses  that  we  are  dealing  with  a  food  relation. 
Multiplication  of  algae  leads  to  increase  of  Anur&a,  which,  in  turn, 
reduces  the  algae,  and  then  itself  declines  until  the  food  planktonts 
again  increase. 

Anurcsa  cochlearis  is  exceedingly  variable  in  the  length  of  the 
posterior  spine,  in  the  development  and  degree  of  curvature  of  the 
anterior  spines,  in  the  arrangement  of  the  areas  of  the  lorica,  and  in 
the  degree  of  its  ornamentation  by  small  spinules.  The  separation 
of  these  varieties  where  every  individual  must  be  assigned  to  some 
one  of  them,  is  a  matter  of  some  difficulty  owing  to  the  presence  of 
intergrading  individuals.  The  characters  which  signalize  var. 
hispida  Lauterborn  and  var.  irregularis  Lauterborn  are  not  quickly 
recognized  under  the  conditions  of  rapid  plankton  enumeration,  and 
no  effort  was  made  to  trace  their  seasonal  distribution  in  our  plank- 
ton. Lauterborn's  var.  macracantha  was  included  with  the  type 
form — his  var.  typica — in  our  records.  These  two  include  those 
individuals  with  medium-sized  and  longer  posterior  spines.  In  our 
waters  the  variety  macracantha  is  relatively  rare,  at  least  as  figured 
by  Lauterborn  ('98).  Indeed,  both  the  type  and  this  variety  consti- 


152 

tute  less  than  a  seventh  of  the  total  representatives  of  the  species. 
Their  distribution  throughout  the  year  (Table  I.)  accords  with  the 
results  obtained  by  Lauterborn  ('98),  who  found  that  the  average 
length  of  the  posterior  spine  from  January  to  May  and  from  October 
to  December  was  from  78  to  48/i,  while  from  June  to  September  it 
was  from  28.5  to  21  //.  In  Table  I.  it  will  be  seen  that  the  longer- 
spined  forms  which  I  have  referred  to  A.  cochlearis  var.  macracantha 
and  var.  typica  occur  in  the  plankton  from  January  to  May  3 1 ,  and 
then  disappear,  returning  again,  in  small  numbers,  October. 25.  The 
short-spined  variety  referred  by  me  to  A.  cochlearis  var.  stipitata 
and  the  spineless  var.  tecta  are,  on  the  other  hand,  continued  during 
the  summer.  The  natural  result  would  be  that  the  average  length 
of  the  spines  in  the  species  as  a  whole  would  fall  during  the  summer 
months.  It  is  apparent  that  this  tendency  on  the  part  of  A. 
cochlearis  to  become  shorter  and  smaller  during  the  summer  months 
does  not  bear  out  the  contention  of  Wesenberg-Lund  ('98)  that 
winter  individuals  are  smaller  and  summer  ones  larger  among 
perennial  rotifers.  He  reports  var.  tecta  as  "die  Hauptform  des 
Winters  "  in  several  Danish  lakes,  and  the  variety  with  a  long 
horn  as  a  summer  form,  found  in  July-August. 

Of  these  varieties,  macracantha,  typica,  and  stipitata  intergrade 
in  our  waters  with  numerous  connecting  links,  while  var.  tecta  is  not 
connected  with  the  other  forms  by  many  individuals  with  inter- 
mediate characters.  Lauterborn  ('98)  also  notes  the  greater  inde- 
pendence of  this  variety  in  the  waters  of  the  Rhine. 

In  Table  I.  the  seasonal  distribution  of  these  three  varieties,  the 
.long-spined  (typica  and  macracantha},  the  short-spined  (stipitata), 
and  the  spineless  (tecta}  are  given  separately.  It  will  be  noted  that 
the  long-spined  form  has  the  distribution  above  mentioned,  that 
var.  tecta  runs  throughout  the  whole  year,  and  that  var.  stipitata  is 
absent  in  midwinter  and  is  a  common  summer  form.  The  relative 
numbers  of  the  varieties  fluctuate  in  different  years.  For  example, 
var.  tecta  was  relatively  but  one  fourth  as  abundant  in  1897  as  in 
1898.  As  shown  in  Table  I.,  whenever  coincidently  present  in  the 
plankton  all  the  varieties  respond  to  the  causes  which  produce  the 
rhythm  of  occurrence,  the  rise,  culmination,  and  decline  of  the  pulses 
being  much  alike  in  all  of  the  varieties. 

About  three  eighths  of  the  females  noted  in  1898  were  ovigerous, 
carrying  as  a  rule  but  a  single  egg.  Instances  of  two  eggs  were 


153 


noted,  but  they  are  rare.  The  greatest  proportion  of  egg-bearing 
females  appears  during  the  rise  of  the  pulse,  as  is  seen  in  the  follow- 
ing table,  which  gives  the  data  of  the  vernal  pulse  in  1898.  From 


ANUR^EA  COCHLEARIS. 


Date 

No.  of 
ovigerous 
females 

Total 

females 

Total 
eggs 

Ratio  of 
eggs  to 
individuals 

No.  of 
dead 

April  12  

800 

2,200 

800 

1     2   75 

0 

April  19  

6,400 

15,200 

8,800 

1      1.73 

400 

April  26  

45   000 

137   800 

65   000 

1      2    12 

3   200 

May  3  . 

536,000 

1,022,400 

552,200 

1      1.85 

9  600 

May  10  

489   600' 

1    145   600 

643   200 

1      1   78 

99  200 

May  17....  

110,400 

434,800 

160  000 

1     2   71 

100  000 

May  24  

6,000 

21,200 

7,200 

1      2.94 

1,800 

May  31.  

3   000 

11    200  * 

3,400 

1      3   29 

1   800 

April  12  to  the  crest  of  the  pulse  on  May  10  (not  inclusive)  the  aver- 
age ratio  of  eggs  to  individuals  was  1  to  1.87.  From  the  crest  to  thp 
foot  of  the  decline  inclusive  the  ratio  is  1  to  2.98.  The  number  of 
empty  loricus  is  given  below,  and  it  will  be  noted  that  on  the  week 
prior  to  the  crest  of  the  pulse  there  wrere  107  living  to  one  dead ;  on 
the  crest  itself,  one  to  twelve ;  while  the  week  following  the  crest  of 
the  pulse  there  was  an  empty  lorica  for  every  4.3  living  females. 
Rapid  multiplication  thus  attends  the  rise  of  the  pulse  and  rapid 
destruction  its  decline.  Parasites  were  very  rarely  observed  in  this 
species.  The  decline  of  a  pulse  is  thus  due  to  the  cessation  of 
reproduction  and  a  relatively  heavy  death  rate. 

Apstein  ('96)  finds  that  in  Lake  Plon  Anuraa  reaches  its  maxi- 
mum in  July  and  is  at  its  minimum  in  April.  It  is  everywhere 
common  in  the  German  wraters.  A.  tecta,  on  the  other  hand,  was 
found  only  in  the  smaller  lakes  and  in  great  numbers,  replacing 
cochlearis  in  warmer  months  to  some  extent.  Lauterborn  ('98) 
regards  it  as  the  most  abundant  rotifer  in  the  Rhine.  Our  statistical 
records  do  not  show  that  this  is  the  case  in  the  Illinois,  for  it  is  here 


154 

surpassed  by  several  other  species.  Zimmer  ('99)  finds  that  this 
species  is  the  most  common  winter  rotifer  in  the  plankton  of  the 
Oder,  with  a  maximum  in  the  spring  and  a  predominance  of  var. 
tecta  from  July  to  September.  Schorler  ('00)  finds  it  to  be  the  most 
common  rotifer  in  the  Elbe — from  April  to  November ;  and  Skor- 
ikow  ('97)  finds  it  in  the  Udy,  in  Russia,  throughout  the  summer 
in  great  numbers,  but  surpassed  by  Synchczta,  Polyarthra,  and 
Brachionus  angularis.  The  variety  tecta  greatly  exceeds  var. 
stipitata  in  these  waters.  Seligo  ('00)  finds  it  throughout  the  year 
in  Prussian  lakes  near  Danzig,  with  a  maximum  in  May.  There  are 
indications,  in  his  data,  of  recurrent  pulses  during  the  summer,  but 
his  interval  of  collection  is  too  great  to  follow  their  history.  Burck- 
hardt  ('OOa)  finds  it  throughout  the  year  in  Swiss  waters,  with  its 
single  maximum  in  August.  Jennings  ('94,  '96,  and  '00)  reports  it 
in  the  summer  plankton  of  Lake  Michigan  and  Lake  Erie  and  of 
inland  waters  of  Michigan. 

Anuraza  hypelasma  Gosse. — Average  number  of  females,  2,390; 
of  eggs,  1,917.  This  species  has  a  very  definite  limitation  to  a 
period  extending  from  early  in  June  to  the  first  days  of  November. 
There  are  but  two  records  outside  of  these  limits — a  single  female  and 
egg  on  Jan.  11,  1898,  and  another  upon  April  19  of  the  same  year. 
The  probabilities  of  occurrence  in  very  small  numbers  at  all  tempera- 
tures is  thus  indicated.  The  following  table  gives  the  data  of  pulses 
and  temperatures. 

All  of  the  pulses  save  one  occur  at  temperatures  above  70°,  and 
with  this  exception  the  species  declines  rapidly  and  disappears 
shortly  after  temperatures  pass  below  60°.  It  is  plainly,  in  our 
waters,  a  summer  planktont,  with  its  optimum  temperature  close 
to  the  summer  maximum.  This  species  takes  no  share  in  the  vernal 
pulse,  and  there  is  no  satisfactory  evidence  of  any  fluctuation 
corresponding  to  it  at  any  other  season.  There  are  three  or  four 
pulses  in  each  summer,  and  the  species  is  apparently  poly  cyclic,  for 
winter  eggs  were  found  in  1898  either  at  the  maximum  of  the  pulse 
or  the  week  or  fortnight  following.  Thus  24,000  winter  eggs  were 
recorded  on  Sept.  27,  1898,  the  date  of  the  maximum  of  the  Septem- 
ber pulse.  The  parthenogenetic  eggs  preponderate  during  the  rise 
of  the  pulses  in  a  very  marked  manner  in  this  species.  For  example, 
in  this  September  pulse  55,400  eggs  were  recorded  during  its  rise 
to  500  during  its  decline.  .  In  like  manner,  in  the  case  of  the 


155 


PULSES  OF  ANUR/EA  HYPELASMA. 


VpvQf 

First 

record 

I 

•Dulses 

Date 

Temp. 

Date 

Temp. 

No. 

1896  

June  2  7 

80° 

June  2  7 

80° 

1  ,200 

1897  

June  28 

75° 

July  14 

79° 

10,400 

1898  

June  14 

83° 

June  2~1 

77° 

9,600 

Year 

Pulses 

Last  record 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

1896 

Aug.  15 
"     29 

81° 
74° 

2,000 
3,600 

Sept.  30 

58° 

1897 

Aug.  31 

80° 

20,000 

Oct.      5 

71° 

23,200 

Nov.  2 

55° 

1898 

Aug.  16 

77° 

16,000 

Sept.  27 
Oct.    18 

73° 
52° 

43  ,  200. 
13,500 

Nov.  1 

45° 

August  pulse  15,200  eggs  were  found  on  the  rise  to  4,000  on  the 
decline. 

The  location  of  the  pulses  of  A.  hypelasma  is  of  special  interest. 
It  will  be  seen  in  Table  I.  that  they  occur  in  1898  in  the  same  col- 
lections in  which  the  pulses  of  the  other  species  of  Anuraa  and  many 
other  rotifers  occur,  or  in  collections  but  a  week  removed.  They 
coincide  in  general  with  dates  of  the  ploiman  maxima  noted  in  the 
opening  discussion,  and  exhibit  the  same  correlation  with  hydro- 
graphic  conditions  and  intercalation  with  the  pulses  of  chlorophyll- 
bearing  organisms  which  were  noted  in  the  general  discussion  and 
have  been  found  in  preceding  species.  The  comparison  with  Anur&a 
of  the  cochlearis  group  affords  a  curious  instance  of  an  entire  sup- 
pression (Table  I.)  of  one  species  of  a  genus  (cochlearis}  in  the  month 
of  August  and  the  occurrence  of  a  normal  pulse  in  another  (hypelas- 
ma}. Comparison  of  the  distribution  of  cochlearis  in  previous 
summers  would  lead  us  to  expect  a  cochlearis  pulse  in  August,  1898, 


156 


but  none  appears  in  this  interval,  while  hypelasma  runs  a  normal 
course  of  recurrent  pulses  throughout  the  summer.  This  August 
pulse  of  hypelasma  (Table  I.)  culminates  August  16,  just  a  week 
after  the  symmetrical  and  well-defined  pulse  of  chlorophyll-bearing 
organisms  (PI.  II.)  of  August  9. 

With  a  single  exception,  all  of  the  pulses  of  1896  and  1897,  indi- 
cated in  the  table,  fall  a  week  later  than,  or  coincide  with,  the  pulses 
of  chlorophyll-bearing  organisms,  as  in  1898t 

This  species  has  not  occupied  a  prominent  place  in  the  literature 
of  fresh-water  plankton.  Weber  ('98)  finds  it  rare  in  Swiss  waters 
in  the  summer.  Lauterborn  ('93)  classes  it  with  the  monocyclic 
summer  forms  in  the  plankton  of  the  Rhine,  though  he  states  in  a 
footnote  that  he  had"  found  winter  eggs  once  in  June.  It  is  probably 
poly  cyclic  in  our  waters.  Skorikow  ('96)  finds  it  in  the  summer 
plankton  of  the  river  Udy,  in  Russia,  but  it  is  not  mentioned  by  other 
investigators  of  the  potamoplankton  of  Europe.  Apstein  ('96)  does 
not  report  it  from  Lake  Plon. 

Asplanchna  brightwellii  Gosse. — Average  number,  of  adults  2,079, 
of  eggs,  396;  averages  in  1897,  16,161  and  2,156.  This  is  a  poly- 
cyclic  perennial  planktont  in  our  waters.  It  has  been  found  in 
every  month  of  the  year,  but  the  greater  numbers  and  more  con- 
tinuous occurrences  lie  between  May  1  and  October  30.  In  1898 
(Table  I.)  all  but  200  of  the  108,120  recorded,  lie  within  these  limits, 
and  all  but  260  above  60°.  In  previous  years  approximately  the 
same  limits  are  found.  The  following  table  gives  the  data  of  pulses 
and  temperatures. 

PULSES  OF  ASPLANCHNA  BRIGHTWELLII. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 

1895 

June  19 
June  27 

80° 
80° 

6,678 
1,600 

1896  

May   1 

70° 

1,788 

1897 

1898  

May   5 

60° 

20,800 

June  21 

77° 

1,100 

157 


PULSES  OF  ASPLANCHNA.  BRIGHTWELLII — Continued. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

•     No. 

Date 

Temp. 

No. 

1894 

July  30 

82° 

19,398 

1895 

July  29 

75° 

1,344 

Aug.  12 

79° 

118,206 

Nov.  14 

45° 

1,725 

1896 

Aug.  2  1 

79° 

1,200 

1897 

July  21 

83° 

3,200 

Aug.  10 

81° 

5,200 

Sept.-  9 

.   80° 

284,000 

1898 

Aug.     2 

79° 

23,200 

Aug.  23 

81° 

4,000 

Sept.  27 

73° 

6,400 

It  will  be  seen  from  this  table  that  all  the  pulses  save  one,  and 
that  one  (Nov.  14,  1895)  poorly  denned,  lie  between  60°  and  the 
maximum  temperatures,  indicating  an  optimum  near  the  summer 
maximum.  There  is  in  this  species  no  prominent  vernal  pulse  such 
as  that  found  in  Anurcea,  and  the  highest  numbers  were  reached 
during  the  height  of  the  warm  season. 

The  evidence  of  the  polycyclic  character  of  the  seasonal  distribu- 
tion of  this  species  is  shown  in  the  following  table,  which  gives 
the  occurrences  of  ovigerous  females,  males,  and  winter  eggs  in  1898. 
It  will  be  noted  that  ovigerous  females  are  more  numerous  during  the 
rise  of  the  pulse;  that  the  males  appear  just  before,  during,  and 
after  the  culmination  of  the  pulse ;  and  that  winter  eggs  are  absent 
only  during  the  rise  of  the  pulse,  and  appear  at  or  after  its  culmina- 
tion and  during  the  decline.  The  data  given  afford  a  fine  illustration 
of  the  seasonal  distribution  of  polycyclic  rotifers,  and  of  the  relation 
of  the  sexual  cycle  to  the  number  and  character  of  the  representa- 
tives of  the  species  in  the  plankton.  The  growth  of  the  pulse  .results 
from  a  rapid  succession  of  parthenogenetic  generations  in  the  course 
of  about  two  weeks,  and  it  culminates  with  or  shortly  after  a  pulse 
in  the  food  supply.  The  decrease  in  food  supply  is  attended  by  the 
appearance  of  males  and  winter  eggs,  a  decrease  in  ovigerous 
females,  and  a  decline  of  the  species.  With  the  recurrence  of  the 
food  supply  the  parthenogenetic  cycle  again  begins.  The  same 
course  of  events  is  run  in  each  recurrent  pulse.  Food  supply 
rather  than  temperature  seems  to  be  the  determining  factor  in  this 
rhythm. 


158 

ASPLANCHNA   BRIGHTWELLII. 


Date 

Males 

Females 
without  eggs 

Ovigerous 
females 

Winter 
eggs 

May    3  

0 

3,200 

12,800 

0 

"      10  

8,000 

4,800 

8,000 

1   600 

"      17       

1   600 

5   600 

4  000 

100 

"24 

400 

0 

200 

"      31 

200 

0 

400 

June    7                       ... 

200 

0 

0 

14 

0 

0 

0 

21           

800 

300 

0 

28 

100 

0 

0 

Tulv    5 

120 

40 

120 

12 

o 

o 

19 

40 

240 

"      26  

240 

12   400 

5   260 

60 

August    2  

4  000 

7   200 

12   000 

5   600 

9 

80 

0 

800 

16  

0 

800 

800 

23 

3   200 

800 

60 

30.  .  .    ... 

1   600 

800 

1   600 

September  6 

o 

0 

0 

13 

o 

0 

0 

20  

540 

600 

0 

27 

3   200 

3   200 

0 

October    4  

500 

0 

1   000 

11 

1   000 

0 

500 

159 

An  examination  of  the  location  of  the  pulses  of  Asplanchna 
brightwellii  shows  (Table  I.)  that  in  1898  one  coincided  with  the 
pulse  of  chlorophyll-bearing  organisms  (PI.  II.)  and  the  remaining 
four  followed  it  either  in  a  week  or  fortnight.  In  previous  years 
two  pulses  coincide  with  and  five  follow  those  of  chlorophyll-bearing 
organisms,  and  a  single  ill-defined  one  (Nov.  14,  1895)  precedes. 

This  species  is  not  wholly  herbivorous  in  its  feeding  habits. 
Codonella,  Difflugia,  and  even  other  rotifers  such  as  Brachionus  and 
Anur<za,&re  frequently  seen  in  the  digestive  tract.  _Diatoms,  even 
Melosira  and  Peridiniidce,  as  well  as  Pediastrum  and  other  algas,  are 
frequently  taken  as  food.  In  one  instance  a  Daphnia  cucullata  300  /* 
in  length  was  seen  in  the  stomach  in  a  transverse  position.  It  was 
fully  a  third  the  length  of  the  animal  which  had  eaten  it. 

Asplanchna  brightwellii  is  reported  by  Skorikow  ('97)  in  the 
summer  plankton  of  the  Udy,  in  Russia ;  by  Schorler  ('00)  as  spo- 
radic in  the  Elbe  in  June  and  September;  and  by  Lauterborn  ('93) 
from  the  Rhine,  where  its  cycle  coincides  with  that  of  A.  priodonta. 
Zacharias  ('98)  reports  it  in  German  reservoirs  in  June  and  August. 
It  is  a  cosmopolitan  species,  but  does  not  seem  to  have  been  found 
by  other  plankton  investigators  in  European  waters. 

Asplanchna  ebbesbornii  Huds. — Average  number  of  adults  in 
1895,  942.  In  1898,  only  winter  eggs  of  the  species  were  noted  in 
the  plankton  in  February,  June,  July,  September,  and  October, 
though  adults  were  doubtless  there.  Adults  have  doubtless  oc- 
curred sporadically  in  all  other  years,  and  in  1895  reach  a  pulse  of 
21,518  on  July  6  at  81°,  which  was  followed  by  the  appearance  of 
males  and  winter  eggs.  All  records  of  adults  lie  between  April  29 
and  September  14  and  above  60°.  This  rare  rotifer  has  not  appeared 
in  the  literature  of  fresh-water  plankton  elsewhere  to  my  knowledge. 
Hempel's  statement  ('99)  that  his  record  of  its  occurrence  in  the 
Illinois  is  the  first  for  this  continent  must  be  modified,  since  Leidy 
('87)  found  it  near  Philadelphia.  It  is  evidently  a  summer  plank- 
tont  in  our  waters,  and  the  wide  distribution  of  its  winter  eggs 
suggests  that  it,  too,  may  be  poly  cyclic  ;  and  their  appearance 
in  the  plankton  in  large  numbers  with  reference  to  the  adults  taken, 
leads  to  the  further  inference  that  its  center  of  distribution  is  prob- 
ably not  in  channel  waters,  and  that  it  may  be  predominantly 
limicolous  species,  or  have  its  center  of  distribution  in  the  quieter 
backwaters. 


160 

Asplanchna  girodi  de  Guerne  is  reported  by  Hempel  ('99)  in  the 
backwaters  in  April. 

Asplanchna  herricki  de  Guerne. — Average  number,  15;  in  1897, 
295  ;  in  1896,  317.  This  species  was  always  rather  rare  in  our  waters, 
and  is  apparently  a  summer  planktont.  The  earliest  record  is 
April  29,  at  64°,  and  the  latest,  November  15,  at  48°.  There  is  an 
indication  of  a  vernal  pulse  in  April-May  in  1896  and  1897,  and  the 
recurrence  of  the  species  at  intervals  of  a  few  weeks  during  the  sum- 
mer suggests  a  polycyclic  habit  similar  to  that  of  other  members  of 
the  genus  in  our  waters,  but  the  data  are  insufficient  to  follow  the 
cycles  if  such  exist.  Ovigerous  females  were  present  when  numbers 
were  greatest,  and  males  and  females  with  winter  eggs  were  found 
at  the  time  of  the  vernal  pulse  on  May  25  (3,200)  in  1897.  Hempel's 
statement  ('99)  of  its  rarity  in  June  and  July  is  not  borne  out  by  the 
statistical  records  in  these  months  in  1896,  1897,  and  1898. 

This  rotifer  is  abundant  in  the  summer  plankton  of  Lake  St. 
Clair,  Lake  Michigan,  and  lakes  of  northern  Michigan  (Jennings,  '94 
and  '96),  and  it  may  be  significant  that  it  reaches  its  greatest  devel- 
opment in  the  Illinois  in  the  spring  at  60°-70°  and  not  during  the 
period  of  maximum  heat.  This  is  about  the  summer  temperature 
of  those  northern  waters.  This  species  has  not  to  my  knowledge 
appeared  in  the  literature  of  European  plankton,  though  it  is  found 
in  European  waters. 

Asplanchna  priodonta  Gosse. — Average  number,  441 ;  winter 
eggs,  7.  This  species  is  much  less  abundant  in  our  waters  than  its 
associate  A.  brightwellii,  being  outnumbered  by  it  five  to  one  in 
1898.  It  is  in  the  Illinois  River  a  summer  planktont  only,  at  least 
so  far  as  the  records  go,  though  reported  elsewhere  as  perennial. 
The  earliest  record  in  any  year  is  April  29, 1896,  at  70°,  and  the  latest 
October  5,  1897,  at  70°,  when  an  unusual  pulse  of  22,000  was  found. 
The  records  are  too  scattered  to  trace  the  seasonal  history.  There 
are  only  indications  of  recurrent  pulses.  In  May,  1898  (Table  I.), 
the  best-defined  pulse  is  recorded.  The  details,  which  conform  in 
the  main  to  the  sequence  noted  in  A .  brightwellii  of  ovigerous  females 
with  summer  eggs  during  the  rise,  with  males  and  winter  eggs  at  and 
after  the  culmination  of  the  pulse,  are  given  in  the  appended  table. 

This  is  the  only  cycle  found  in  this  year.  The  presence  of 
ovigerous  females  and  winter  eggs  at  other  seasons  as  well,  in  other 


161 

ASPLANCHNA    PRIODONTA. 


Date 

Males 

Females 
without 
eggs 

Females 
with  sum- 
mer eggs 

Females 
with  winter 
eggs 

Total 
individuals 

May  10 

3   200 

3   200 

"      17 

800 

10  400 

3   200 

14  400 

"      24  

120 

1,600 

200 

200 

2,120 

"      31 

1   600 

400 

2   000 

years,  leads  us  to  infer  that  the  species  may  be  poly  cyclic  in  our 
waters. 

This  limnetic  rotifer  figures  largely  in  the  fresh-water  plankton 
of  other  localities,  attaining  a  relative  development  greatly  surpass- 
ing that  thus  far  found  in  the  Illinois  River.  Apstein  ('96)  reports 
it  of  irregular  occurrence  in  the  smaller  lakes  of  Holstein,  and  Seligo 
('00)  finds  it  perennial  in  Prussian  lakes,  with  maxima  in  April  and 
September.  Wesenberg-Lund  ('00)  also  finds  it  perennial  in  Danish 
waters,  with  sexual  cycles  in  May  and  September.  Marsson  ('00),  in 
waters  about  Berlin  finds  a  great  variation  in  the  seasonal  occurrence, 
but  the  intervals  of  his  collection — four  to  six  weeks — were  too  great . 
to  follow  seasonal  distribution  satisfactorily.  Zacharias  ('98b)  finds 
it  in  the  summer  and  autumn  plankton  in  a  number  of  German 
lakes  and  streams.  Zimmer  ('99)  traces  its  appearance  in  the  Oder 
from  February  to  a  maximum  in  May,  from  which  time  until  the  end 
of  July  it  is  "einer  der  haufigsten  Planktonorganismen"  (!).  It 
then  declines,  but  returns  in  small  numbers  in  November.  Schorler 
('00)  records  it  in  the  Elbe  from  April  to  October,  with  maxima  in 
April-June  and  September.  Burckhardt  ('OOa)  finds,  on  the  other 
hand,  that  in  Swiss  waters  it  reaches  its  greatest  development  from 
December  to  March  with  a  maximum  in  January-February.  There 
are  also  secondary  maxima  in  May- June  and  in  August.  Lauter- 
born  ('93)  finds  it  to  be  a  dicyclic  perennial  planktont  in  the  Rhine, 
with  maxima  in  April  and  September-October.  A  part  of  the  great 
variation  in  the  seasonal  distribution  of  this  species  which  is  ap- 
parent in  this  survey  of  the  literature  may  be  due  to  insufficient 
collections  or  too  great  an  interval  between  collections.  The  species 


162 

is  probably  a  polycyclic  planktont  with  its  greater  pulses  in  spring 
and  fall. 

'  Asplanchnopus  myrmcleo  Ehrbg. — Taken  in  small  numbers  and 
irregularly  from  May  to  October  at  temperatures  above  60°. 

Ascomorpha  ecaudis  Perty. — Found  rarely  in  early  summer,  in 
temperatures  above  60°. 

Brachionus. 

The  discussion  of  the  species  of  this  genus  in  our  plankton 
is  fraught  with  great  difficulty.  The  genus  is  represented  in  the 
Illinois  River  by  a  very  large  number  of  individuals  (fully  25 
per  cent,  of  the  total  Ploima),  and  the  species  are,  almost  without 
exception,  exceedingly  variable.  They  are  loricate  forms,  and  the 
variations  affect  the  proportions  of  the  lorica  and  the  development 
of  its  prolongations  in  spines,  antlers,  and  various  diversifications 
of  its  surface.  They  are  evident  upon  the  most  cursory  examination 
in  most  cases,  and  have  been  utilized  by  systematists  for  the  estab- 
lishment of  species.  For  example,  Weber  ('98)  lists  no  less  than  67 
species  of  Brachionus,  the  most  of  which  he  regards  as  synonyms,  and 
he  includes  only  a  part  of  the  species.  Fuller  knowledge  of  the 
extreme  variability  in  this  genus  has  led  the  most  thorough  students 
of  the  rotifers  to  regard  many  of  these  so-called  species  as  but 
varieties  at  the  best,  and  to  express  their  opinion  with  unmistakable 
plainness  that  descriptions  of  new  species  among  rotifers  should  only 
be  made  after  most  careful  determination  of  the  variability  of  the 
organism  (cf.  Rousselet,  '02,  Jennings,  '00,  Wesenberg-Lund,  '00, 
and  Weber  '98). 

For  one  not  a  specialist  in  rotifers,  the  attacking  of  the  Brachionus 
problem  from  the  statistical  standpoint  is  made  difficult  by  the 
coiidition  of  the  literature  of  the  subject,  owing  largely  to  the  semi- 
tropical  distribution  of  the  genus;  by  the  absence  of  any  critical 
monograph  of  the  whole  genus  dealing  fully  with  the  synonymy  of 
the  subject;  and  by  the  necessity  of  establishing  and  maintaining 
constantly  amid  the  ceaseless  change  of  varying  forms  the  same 
standards  of  distinction  between  the  species  or  varieties  into  which 
all  of  the  individuals  enumerated  must  be  assorted.  Furthermore, 
these  distinctions  must  be  established  before  the  plankton  is 
counted ;  that  is,  before  the  limits  of  variation  are  fully  appreciated. 
It  is  needless  to  say  that  my  efforts  are  at  best  but  approximations 


163 

to  a  satisfactory  analysis  of  the  genus  in  our  waters.  Brachionus 
contains  by  virtue  of  variation  in  the  hard  parts  of  its  lorica  most 
excellent  material  for  the  study  of  the  problem  of  variation,  and  its 
rapid  multiplication  makes  possible  a  correlation  with  seasonal  and 
environmental  changes  not  often  afforded. 

Evidence  has  accumulated  in  the  various  papers  of  Schmarda, 
Ehrenberg,  Barrois,  v.  Daday,  Anderson,  and  others  who  have  dealt 
with  the  microscopical  fauna  in  tropical  regions,  that  this  genus 
attains  its  greatest  development  in  the  warmer  waters ._  It  is  there- 
fore not  strange  that  Skorikow  ('96)  finds  the  genus  well  represented 
in  the  warm  and  shallow  waters  of  Russia,  and  that  the  plankton  of 
the  Illinois  River  and  its  backwaters  should  contain  a  large  and 
varied  representation  of  the  genus. 

For  convenience  in  treatment  I  have  arranged  the  individuals  of 
Brachionus  under  the  following  species,  without,  however,  intending 
to  indicate  thereby  that  they  have  equal  claims  for  specific  recogni- 
tion. The  most  of  these  include  one  or  more  varieties,  and  in  desig- 
nating the  varieties  I  have  taken  those  forms — for  example,  in 
Brachionus  bakeri — whose  descriptions  most  closely  fit  the  predomi- 
nant varieties  in  our  waters,  designating  them  often  without  com- 
plete consideration  of  all  synonymic  possibilities.  In  some  cases 
several  possible  varieties  have  been  included  under  one  head.  The 
following  is  the  list  of  species  with  the  varieties  which  have  been 
thus  separately  enumerated. 

Br'achionus  angularis  Gosse, 

var.  bidens  Plate 
bakeri  Ehrbg. 

"      var.  bidentatus  Anderson 
"     brevispinus  Ehrbg. 
"     cluniorbicularis  Skorikow 
"    melhemi  Barrois  and  v.  Daday 
"    obesus      " 
"    rhenanus  Lauterborn 
"     tuber culus  Turner 
budapestinensis  v.  Daday 
militaris  Ehrbg. 
mollis  Hempel 
pala  Ehrbg. 

(12) 


164 

Brachionus  pala  var.  amphiceros  Ehrbg. 
"    dorcas  Gosse 

"      forma  spinosus  Wierz. 
quadratus  Rousselet 
urceolans  Ehrbg. 

var.  rubens.  Ehrbg. 

"     bursarius  Barrois  and  v.  Daday 
variabilis  Hempel 

Brachionus  angularis  Gosse. — Average  number  of  females, 
57,890;  of  males,  25;  of  summer  eggs  carried,  29,560;  of  winter 
eggs,  1,223  ;  of  male  eggs,  54.  Of  the  individuals,  13,973  belong  to 
var.  bidens  and  43,942  to  the  type;  of  the  eggs,  2,035  belong  to  the 
variety  and  28,802  to  the  type. 

The  combined  statistics  of  the  species  will  be  discussed  before 
the  type  and  variety  receive  separate  treatment.  This  species  was 
found  in  every  month  of  the  year  and  throughout  the  whole  range 
of  temperatures,  but  the  period  of  continuous  presence  and  large 
numbers  lies  definitely  between  May  1  and  November  1  and  above 
60°.  In  fact,  in  1898,  98.6  per  cent,  of  all  the  individuals  were  found 
between  May  31  and  October  4  and  above  70°.  Approximately  the 
same  conditions  are  found  in  previous  years  save  in  1896,  when  an 
earlier  spring  (cf.  PL  X.  and  XII.,  Pt.  I.)  is  attended  by  an  earlier 
appearance  of  this  species.  Temperature  seems  thus  to  have  a  very 
decided  effect  upon  the  seasonal  distribution  of  the  species,  and  may 
have  something  to  do  with  its  apparent  absence  in  the  cooler  waters 
of  our  Great  Lakes  and  of  L.  St.  Clair,  for  in  spite  of  all  the  work 
done  upon  rotifers  in  those  regions  by  Jennings  it  has  been  found  but 
once — by  Kellicott  ('97)  in  a  cove  at  Sandusky.  This  identification 
may  be  questionable,  since  he  says  "  I  at  first  took  it  for  B.  mollis 
Hempel."  Notops  pelagicus,  since  described  by  Jennings  ('00),  is 
found  in  the  plankton  of  Lake  Erie,  and  according  to  him  this 
species  is  much  like  B.  mollis  in  its  appearance.  In  any  event  B. 
angularis  is  very  abundant  in  our  warm  waters  and  practically 
absent  in  the  more  northerly  waters  of  Michigan,  whose  summer 
temperatures  are  10°-15°  below  that  of  the  Illinois  River  and  its 
backwaters. 

Brachionus  angularis  presents  the  usual  phenomenon  of  recurrent 
pulses,  but  in  spite  of  the  large  numbers  they  are  rather  less  regular 


165 


than  usual — for  example,  than  those  of  Annraza  (Table  I.).  This 
irregularity  is  somewhat  more  pronounced  in  the  separated  records 
of  the  type  and  variety  (Table  I.)  than  in  their  combined  statistics. 
This  fact  that  their  combined  curve  of  occurrence  is  more  regular 
than  their  separated  curves  constitutes,  to  my  mind,  evidence  that 
we  are  dealing  only  with  one  genetic  cycle,  and  that  the  variety  does 
not  belong  to  a  fully  separated  genetic  series. 

The  following  table  gives  the  data  of  pulses  and  temperatures  in 
the  several  years. 

PULSES  OF  BRACHIONUS  ANGULARIS  INCLUDING  VAR.  BIDENS. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 
1896 

1897 
1898 

July  13 
July  23 

July  10 
"      23 

July  21 
July  19 

82° 
80° 

80° 
80° 

83° 
84° 

12,118 
100,826 

51,200 
53,400 

70,400 
335,600 

July     6 
June  17 

June  28 
June  28 

81° 
76° 

75° 
78° 

399,196 
60  ,  800 

75,000 
544,000 

May  25 

70° 

67,600 

June    7 

78° 

4,800 

Av'g 

1 

36,200 

269,776 

103,924 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 
1896 

Sept.  17 
Aug.  29 
Aug.  21 

73° 
80° 
79° 

1,272 
105,735 
29,600 

Aug.  12 
Aug.     8 

80° 
85° 

585,090 
20,800 

Sept.  16 

71° 

5,051 

1897 

Aug.  31 

80° 

988,000 

Sept.  14 

83° 

368,800 

Oct.    -5 

71° 

18,400 

1898 

Aug.  16 

77° 

353,600 

Sept.    6 
"      27 

79° 
73° 

163,200 
494,400 

Oct.    25 

48° 

11,500 

Av'g 

486,872 

195,834 

11,650 

It  will  be  noted  that  all  the  pulses  with  one  exception  lie  above 
70°,  averaging  in  fact  78.25°,  indicating  an  optimum  temperature 


166 

near  the  summer  maximum.  The  location  of  the  pulses  with  respect 
to  those  of  the  chlorophyll-bearing  organisms  (PL  II.)  shows  in  the 
main  the  same  relation  that  has  been  observed  in  other  ploiman 
rotifers.  In  1895,  three  angularis  pulses  lie  in  the  period  common 
to  both,  one  of  these  coinciding  in  location  and  two  following  at  the 
next  collection.  In  1896,  two  coincide  and  five  follow  at  the  next 
collection  or  shortly  thereafter.  In  1897,  four  follow  at  an  interval 
of  a  week  or  a  fortnight,  and  one  is  located  where  data  are  incom- 
plete. In  1898,  three  coincide  and  three  follow  at  a  short  interval, 
and  one  (June  7),  a  minor  and  ill-defined  pulse,  appears  to  lie  on  the 
rise  of  the  pulse  of  the  chlorophyll-bearing  organisms.  In  the  main 
the  dependence  of  these  rotifer  pulses  upon  the  recurrent  periods  of 
increase  in  these  primal  links  in  the  food  cycle  is  suggested  by  this 
coincidence  or  sequence.  The  pulses  of  Brachiomis  angularis  co- 
incide in  the  main  with  those  of  the  totals  of  ploiman  rotifers 
(Table  I.). 

There  is  no  vernal  pulse  in  the  species  at  the  time  of  the  April- 
May  volumetric  maximum,  and  no  large  autumnal  pulse.  The  pulses 
in  August-September,  at  the  close  of  our  period  of  maximum  heat, 
average  much  greater  than  those  of  other  months,  and  still  further 
indicate  the  relation  of  this  species  to  the  higher  temperatures. 

The  eggs  are  carried  by  the  female  attached  to  the  posterior  end 
of  the  lorica.  Usually  but  a  single  summer  egg  is  carried  at  one 
time,  but  often  two,  three,  and  even  four,  have  been  seen  during  the 
height  of  the  period  of  rapid  reproduction.  The  relation  of  the 
number  of  eggs  to  the  pulses  is  obscured  in  this  species  to  some 
extent  by  the  fact  that  the  eggs  are  similar  to  those  of  other  Brachio- 
nus  and  when  detached  cannot  be  identified  with  certainty.  Records 
are  therefore  based  upon  attached  eggs  only.  The  number  of  these 
depends  to  some  extent  on  the  detachment  in  the  processes  of 
collection,  killing,  and  subsequent  handling.  In  a  few  cases  de- 
tached male  or  winter  eggs  could  be  identified  with  some  degree  of 
probability  by  the  constitution  of  the  rotiferan  plankton.  An 
examination  of  the  records  of  eggs  (Table  I.)  will,  however,  suffice  to 
indicate  the  prevalence  of  rapid  reproduction  during  the  rise  of  the 
pulses  and  the  decline  in  the  process  during  the  fall  of  the  pulse. 
Males,  male  eggs,  and  winter  eggs  were  recorded  in  a  number  of 
instances  at  the  culmination  or  during  the  decline  of  a  pulse.  For 
example,  in  1898,  they  followed  the  pulses  of  August  16,  September 


6,  and  especially  that  of  September  27,  when  they  were  found  con- 
tinuously for  a  month. 

The  separate  records  of  the  type  and  the  variety  (Table  I.)  contain 
in  their  seasonal  distribution  one  point  of  special  interest;  namely, 
the  appearance  of  the  variety  after  the  type  has  been  present  for 
some  time.  An  examination  of  the  records  in  the  several  years 
reveals  the  fact  that  var.  bidens  is  practically  confined  so  far  as 
large  numbers  are  concerned  to  the  months  of  July-September.  This 
appears  in  1898  (Table  I.)  and  is  equally  evident  in"  1&9 6  and  1897, 
but  is  less  noticeable  in  1895.  The  first  large  pulse  is  passed  in  each 
year  before  var.  bidens  takes  any  appreciable  part  in  the  genesis  of 
the  pulses.  Even  the  second  large  pulse  is  not  extensively  con- 
tributed to  by  the  variety  in  some  instances.  On  the  other  hand, 
the  later  pulses  in  1895  and  1897  were  mainly  of  the  variety.  There 
is  thus  in  this  species  some  evidence  of  a  tendency  on  the  part  of  the 
variety  marked  by  the  development  of  a  pair  of  posterior  spines  to  appear 
in  the  latter  part  of  the  period  of  seasonal  occurrence. 

The  variety  bidens  in  our  records  includes  individuals  with  well- 
developed  spines  (B.  caudatus  Barrois  and  v.  Daday),  but  they  are 
not  to  my  mind  worthy  even  of  varietal  distinction,  since  they 
intergrade  so  completely  with  var.  bidens  and  are  merely  well-de- 
veloped examples  of  this  variety,  and  I  see  no  reason  for  giving  the 
variety  two  names. 

Wesenberg-Lund  ('00)  has  expressed  the  opinion  that  the  elonga- 
tion of  structural  processes  which  he  has  noted  in  summer  planktonts 
is  an  adaptation  on  their  part  to  the  changes  in  the  buoyancy  of  the 
water  dependent  upon  changes  in  its  specific  gravity  and,  as  shown 
byOstwald  ('03  and'03a),  in  its  molecular  friction  caused  by  seasonal 
fluctuations  in  temperature.  It  would  seem  that  this  tendency  on 
the  part  of  the  spinous  form  of  Brachionus  angularis  to  appear  in 
greater  proportions  in  late  summer  at  the  period  of  maximum  heat 
in  our  waters  might  be  an  illustration  of  Lund's  thesis  and  Ostwald's 
theoretical  considerations.  The  changes  in  temperature  during  the 
occurrence  of  the  species  are,  however,  not  very  great,  though  our 
incomplete  records  suggest  (Pt.  I.,  Table  III.  and  PL  X. — XII.)  that 
August  temperatures  are  higher  on  an  average  than  those  of  July. 
The  averages  for  June,  July,  and  August  are  77.75°,  81,03°,  and 
81.49°.  In  1897,  the  dominance  of  the  spinous  type  extends  well 
into  September,  but  it  accompanies  a  period  of  summer  heat  (Pt.  I., 


168 

PI.  XI.)  prolonged  for  a  fortnight  into  September,  with  river  water 
at  or  above  80°.  In  1898,  it  falls  away  in  numbers  more  rapidly 
than  the  spineless  form  (Table  I.)  as  temperatures  fall  in  October, 
though  this  tendency  is  less  marked  in  previous  years. 

Brachionus  angularis,  as  above  stated,  seems  to  be  rare  in  the 
plankton  of  our  more  northerly  and  cooler  American  waters.  It  is 
also  conspicuously  absent  from  plankton  of  Swiss  waters,  as  reported 
by  Weber  ('98)  and  Burckhardt  ('00  and  '00a),and  from  German 
lakes  examined  by  Apstein  ('96),  Zacharias  ('98),  and  Seligo  ('00), 
and  from  Finland  waters  examined  by  Stenroos  ('98).  It  was, 
however,  found  by  Wesenberg-Lund  ('98)  in  Danish  waters,  and  in 
the  Udy  River, in  Russia, by  Skorikow  ('97),  whose  statistical  records 
show  it  to  be  the  most  abundant  Brachionus  in  that  stream,  and 
outnumbered  among  the  rotifers  only  by  Synchceta  stylata  and 
Polyarthra.  Schorler  ('00)  finds  it  in  the  Elbe  from  April  to  July 
and  most  abundantly  in  June.  Lauterborn  ('98)  reports  it  as 
perennial  in  the  Rhine  and  poly  cyclic,  with  winter  eggs  in  April, 
June,  August,  October,  and  November.  This  distribution  is  much 
like  that  in  the  Illinois  River,  and  will  probably  be  found  in  tem- 
perate waters  wherever  the  seasonal  cycle  is  thoroughly  examined. 

Brachionus  bakeri  Ehrbg. — Average  number  of  females,  including 
all  varieties,  594 ;  eggs,  420.  The  following  table,  giving  the  average 
of  each  of  the  varieties  in  the  several  years,  will  serve  to  indicate 
their  relative  abundance,  the  totals  showing  the  relative  abundance 
of  the  bakeri  group  in  each  year  and  of  each  variety  in  the  total  of 
all  the  collections. 

Though  the  species  is  greatly  diversified  by  variation  the  number 
of  individuals  is  much  less  than  that  of  many  other  plankton  rotifers 
in  which  variation  is  much  less  apparent. 

It  will  be  noted  that, the  species  was  apparently  more  abundant 
in  the  earlier  years.  This  is  only  in  part  the  result  of  the  distribu- 
tion of  the  collections,  as  is  shown  by  the  fact  that  the  numbers  taken 
were  much  larger.  Thus  in  1898  the  largest  record  is  7,600 ;  in  1897 
there  are  three  occurrences  in-excess  of  this;  in  1896,  two;  in  1895, 
three;  and  in  1894,  four.  The  largest  occurrence,  122,958,  was  on 
June  30,  1894.  The  largest  numbers  by  far  were  recorded  in  1894, 
a  year  of  low  water  in  spring.  The  hydrographic  conditions  of  the 
following  year  were  somewhat  similar,  but  the  development  of 
B.  bakeri  was  much  reduced,  at  least  at  the  time  of  the  collections. 


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170 


The  reducing  effect  of  the  recurrent  floods  of  1896  may  be  traced 
in  the  smaller  numbers  recorded  in  this  year;  and  the  larger  num- 
bers of  1897  may  be  referred  to  the  more  stable  conditions  then 
prevailing.  The  very  small  numbers  of  1898  may  also  be  due  to 
disturbed  hydrographic  conditions  of  that  year.  The  number  is 
much  smaller  than  in  1896,  when  the  hydrograph  was  even  more 
disturbed,  but  in  this  latter  year  there  was  more  run-off  of 
impounded  backwaters  during  the  occurrence  of  B.  bakeri,axuA  this 
would  tend  to  favor  their  appearance  in  channel  waters. 

The  occurrences  and  numbers  of  this  species  (as  a  whole)  are 
everywhere  somewhat  irregular,  so  that  pulses  of  occurrence  are 
somewhat  ill  defined.  Several  such  pulses  are  indicated  in  1898, 
and  others  recur  in  the  records  of  previous  years.  As  suggested  by 
the  data  of  1898  (Table  I.),  the  several  varieties  share  in  these  pulses. 
The  evidence  upon  this  point  is  much  more  striking  in  other  years, 
when  numbers  are  larger.  For  example,  in  the  following  table  note 
the  pulse  of  26,800  on  August  23,  1897. 


o 

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to 

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Aug.  10  

0 

o 

0 

0 

o 

200 

200 

"      18  

0 

1,200 

200 

600 

2   000 

5   200 

7  400 

"      23  

o 

7   800 

1   800 

3   400 

2   200 

11   600 

26   800 

"      28  

0 

200 

400 

200 

0 

1   000 

1   800 

In  their  location  these  pulses  exhibit  as  a  rule  the  same  relation  of 
coincidence  or  sequence  to  the  pulses  of  chlorophyll-bearing  organ- 
isms noted  in  some  other  species,  and  they  frequently  coincide  with 
those  of  other  Ploima,  but  not  always. 

This  is  perhaps  the  most  variable  of  the  rotifers  of  the  plankton. 
At  least  its  variations  affect  the  fixed  processes  of  the  lorica  and  are 
thus  quickly  and  easily  appreciated.  The  species,  in  common  with 


171 

B.  pala,  B.  angularis,  and  probably  B.  urceolaris,  has  a  variety — in 
fact,  several  varieties — with  two  posterior  spines  which  are  usually 
symmetrically  placed  but  not  always  symmetrically  developed.  The 
form  without  posterior  spines  (var.  dumorbicularis  Skorikow)  inter- 
grades  with  these,  and  a  series  might  be  formed  with  complete 
intergradations  linking  this  in  turn  with  var.  rhenanus  Lauterborn, 
in  which  the  spines  are  but  slightly  and  often  unequally  developed. 
From  this  we  pass,  by  a  slight  elongation  of  the  posterior  spines,  to 
var.  brevispinus  Ehrbg.,  thence  to  the  type  in  which- the  spines  as 
figured  by  Rousselet  ('97)  are  directed  .posteriorly  with  but  slight 
curvature.  From  this  we  may  pass  toward  variants  in  which  the 
symmetry  is  preserved,  but  the  spines  are  much  elongated  and 
curved  outwardly.  The  anterior  spines  in  such  individuals  are  also 
more  elongated  and  exhibit  a  similar  outward  curvature  (var. 
melhemi  Barrois  and  v.  Daday).  Extreme  types  of  this  curvature 
sometimes  occur  (J5.  falcatus  Zach.).  In  another  direction  we  find 
the  bilateral  symmetry  of  the  processes,  both  anterior  and  posterior, 
to  some  extent  lost  as  a  result  of  differences  in  the  curvature  of  the 
spines  (var.  tuberculus  Turner).  There  are  also  differences  in  the 
surface  markings  of  the  lorica  which  have  been  utilized  as  specific 
distinctions.  Kertesz  ('94)  describes  as  B.  granulatus  a  species 
with  a  minutely  pustulate  surface,  and  Turner's  B.  tuberculus  takes 
its  name  from  this  same  feature.  It  seems  questionable,  however, 
if  these  surface  markings  are  even  of  varietal  value.  Individuals 
without  spines,  in  which  the  transverse  diameter  is  relatively  large 
(var.  obesus  Barrois  and  v.  Daday) ,  are  also  found. 

In  assorting  the  individuals  belonging  to  this  variable  group  I 
have  arranged  them  under  the  following  heads:  bakeri  O.  F.  Mull., 
bidentata  Anderson  (non  bidentatus  Kertesz),  brevispinus  Ehrbg., 
cluniorbicularis  Skor.,  melhemi  Barrois  and  v.  Daday,  obesus  Barrois 
and  v.  Daday,  rhenanus  Lauterborn,  and  tuberculus  Turner.  The 
number  might  have  been  increased.  The  individuals  referred  to 
var.  melhemi  include  many  if  not  all  of  the  long-spined  specimens 
such  as  Rousselet  ('97)  has  referred  to  the  type,  the  latter  designa- 
tion having  been  given  to  individuals  intermediate  between  this  and 
brevispinus.  The  variety  tuberculus  includes  the  asymmetrical 
individuals,  regardless  of  the  surface  markings.  I  will  now  briefly 
compare  the  seasonal  distribution  of  these  varieties  and  note 
any  peculiarities  which  mark  them  individually : — 


172 

Brachionus  bakeri  O.  F.  Mull.,  type  form. — Average  number,  2. 
As  shown  in  table  on  p.  1 93  (MS.),  this  form  is  much  more  abundant 
in  previous  years  though  it  is  relatively  rare,  ranking  sixth  in  the  list 
of  seven  forms  recognized.  The  most  of  the  records  fall  prior  to  the 
middle  of  August,  and  it  seems  to  be  an  early  rather  than  a  late 
summer  form. 

Brachionus  bakeri  var.  obesus  Barrois  and  v.  Daday. — Average 
number  of  females,  41 ;  of  eggs,  62.  The  proportion  of  egg-bearing 
to  non-egg-bearing  females — 2  to  3  in  all  records — is  larger  than  in 
any  other  variety.  It  seems  probable  that  the  lateral  expansion 
which  marks  this  variety  may  be  only  the  result  of  rapid  reproduc- 
tion. In  common  with  most  of  the  other  varieties  this  one  occurs 
at  the  time  of  the  pulses,  but  it  is  last  in  the  list  of  seven,  and  the 
numbers  are  too  small  to  trace  its  seasonal  preferences  with  cer- 
tainty. 

Brachionus  bakeri  var.  bidentatus  Anderson  (non  Kertesz). — 
Found  once— August  5,  1895,  at  78°. 

Brachionus  bakeri  var.  cluniorbicularis  Skor. — Average  number 
of  females,  90;  of  eggs,  95.  This  also  was  more  abundant  in  all 
previous  years.  This  variety  is,  next  to  tuber culus,  the  most 
abundant  of  the  varieties  in  our  plankton.  The  two  stand  at 
opposite  extremes  of  the  series  of  varieties,  the  former  being  least 
modified,  and  the  latter  most,  especially  in  the  direction  of  asym- 
metry. It  includes  about  one  third  of  all  the  individuals  of  the 
species.  The  ratio  in  the  grand  total  of  females  to  eggs  carried— 
11,708  to  5,976 — is  somewhat  less  than  the  average  in  the  entire 
species.  This  variety  is  distributed  throughout  the  whole  seasonal 
range  of  the  species  with  no  marked  predominance  in  any  particular 
part  of  it.  It  is  wholly  absent  in  the  early  summer  of  1 897 ,  but  very 
abundant  in  late  summer  of  that  year,  though  not  in  other  years. 
The  autumn  of  1897  was  one  of  long-continued  high  temperatures 
(Pt.  I.,  PL  XL),  and  under  those  conditions  this  variety  constituted 
two  thirds  of  the  individuals  belonging  to  the  species.  If  we  add  to 
it  the  representatives  of  rhenanus,  obesus,  and  brevispinus  we  have 
a  total  of  15,400  individuals  with  no  posterior  spines,  or  with  spines 
but  slightly  developed,  in  contrast  with  only  2,200  with  such  well- 
developed  spines  referred  to  varieties  melhemi  and  tuberculus.  The 
conditions  of  temperature  were  those  in  which  according  to  the 


173 

hypothesis  of  Wesenberg-Lund  ('00)  we  should  expect  a  predomi- 
nance of  the  long-spined  forms. 

Brachionus  bakeri  var.  rhenanus  Laut. — Average  number  of 
females,  118;  of  eggs,  138;  but  more  abundant  in  previous  years. 
This  is  the  third  in  numbers  on  the  list  of  seven  varieties,  being 
surpassed  only  by  duniorbicularis  and  tuberculus.  It  includes  about 
one  sixth  of  the  individuals  referred  to  this  species.  '  It  is  found 
throughout  the  whole  range  of  the  seasonal  distribution  of  the 
species  and  exhibits  the  same  peculiarities  noted  in-cluniorbicularis, 
to  which  it  is  very  closely  related.  The  proportion  of  females  to 
eggs  noted  in  this  variety  is  very  large;  5,284  to  5,485  in  the  grand 
total. 

Brachionus  bakeri  var.  brevispinus  Ehrbg. — Average  number  of 
females,  795 ;  of  eggs,  390;  but  somewhat  more  abundant  in  previ- 
ous years.  It  was  found  throughout  the  whole  seasonal  range  of 
the  species,  but  not  quite  so  abundantly  in  the  latter  as  in  the  earlier 
half  of  the  summer,  resembling  in  this  particular  the  type.  The 
number  of  eggs  carried  in  this  species  is  in  relation  to  the  number  of 
females  less  than  usual — 3,906  to  795. 

Brachionus  bakeri  var.  melhemi  Barrois  and  v.  Daday. — Average 
number  of  females,  49;  of  eggs,  49.  More  abundant  in  previous 
years,  especially  in  1894,  when  it  constituted  over  a  fifth  of  the 
individuals  (25,764)  in  the  largest  pulse  recorded  for  the  species  as  a 
whole — 122,958  on  July  30.  In  the  aggregate  in  all  years  it  includes 
only  about  a  ninth  of  the  individuals  referred  to  the  species.  This 
form  was  originally  described  from  Syria,  but  it  is  found  in  great 
perfection  in  our  plankton,  even  in  the  extreme  type  described  by 
Zacharias  ('98b)  as  B.  falcatus.  It  occurs  throughout  the  whole 
seasonal  range  of  the  species,  its  distribution  being  somewhat  similar 
to  that  of  tuberculus.  I  do  not  find  any  constant  tendency  limiting 
its  occurrence  to  any  part  of  the  seasonal  range. 

Brachionus  bakeri  var.  tuberculus  Turner. — Average  number  of 
females,  155  ;  of  eggs,  42  ;  but  very  much  more  abundant  in  previous 
years,  especially  in  1894,  when  it  constituted  almost  half  (55,332)  of 
the  largest  pulse  of  the  species  (122,958).  This,  the  most  divergent 
of  all  the  varieties,  constitutes  over  a  third  of  all  the  individuals 
referred  to  the  species.  It  occurs  throughout  the  whole  seasonal 
range  of  the  species,  though  the  larger  numbers  were  found  in 
1894-97  in  the  earlier  part  or  middle  of  the  summer.  I  find  nothing 


174 

in  a  comparison  of  the  seasonal  distribution  of  these  more  decidedly 
spinous  varieties  of  B.  bakeri  with  that  of  the  smoother  forms,  such 
as  cluniorbicularis ,  which  indicates  any  correlation  with  temperature 
conditions  of  a  nature  to  support  Wesenberg-Lund's  suggestion 
that  the  elongation  of  the  processes  of  plankton  organisms  arises  in 
response  to  the  lessened  buoyancy  of  the  water  during  higher  tem- 
peratures. Forms  with  and  without  such  processes  are  found  among 
the  varieties  of  this  species,  and  both  occur  indiscriminately  through- 
out the  whole  range  of  seasonal  occurrence,  and,  so  far  as  I  can  see, 
the  statistical  data  of  their  distribution  with  respect  to  temperature 
afford  no  evidence  of  a  correlation  of  spinosity  and  high  tempera- 
tures in  this  species.  Other  factors  doubtless  enter  into  this 
problem  and  obscure  this  response  if  it  exists. 

B.  bakeri  is  everywhere  widely  distributed  in  fresh  water.  Its 
occurrence  in  the  plankton  of  open  waters  has  not,  however,  been  a 
matter  of  frequent  note.  In  fact  there  is  some  reason  to  think  that 
it  is  largely  confined  to  shallow  warm  waters  where  vegetation  is 
close  at  hand,  or  where  at  least  the  flagellates  and  smaller  algae 
abound,  as  they  do  in  water  fertilized  by  decaying  vegetation  or 
other  organic  matter.  There  is,  it  seems,  no  reason  for  regarding 
this  species  as  merely  adventitious  in  our  plankton.  It  bears  all  the 
characteristics  of  a  true  limnetic  organism  in  our  environment.  Its 
presence  in  the  plankton  is  not  due  to  floods  or  other  disturbances 
which  might  carry  it  from  a  littoral  region  into  the  open  water.  It 
exhibits  characteristic  pulses,  and  is  found  everywhere  in  summer 
in  company  with  typical  planktonts  in  open  water. 

Zacharias  ('98)  records  it  in  some  German  ponds  and  streams, 
and  Weber  ('98)  in  Swiss  marshes  in  the  warmer  months.  Stenroos 
('98)  also  finds  it  in  the  summer  plankton  of  littoral  and  open  waters 
in  the  shallow  Nurmijarvi  Lake  in  Finland.  Jennings  ('00)  reports 
it  as  one  of  the  commonest  rotifers  in  East  Harbor,  Lake  Erie,  and 
in  the  swamps  on  the  islands.  In  land-locked  pools  short-spined 
varieties  were  found,  and  in  swamps  the  long-spined.  Speaking  of 
this  difference,  Jennings  says  "  Possibly  the  different  form  found  in 
these  pools  is  due  to  the  greater  concentration  of  various  salts  in  this 
water  or  to  some  kindred  factor."  In  our  own  region  both  varieties 
occur  at  the  same  time  in  the  same  environments,  channel  and 
backwaters  alike,  and  such  factors  as  Jennings  suggests  to  explain 
the  appearance  of  the  varieties  cannot  well  be  operative  here  in 


175 

channel  waters.  Schorler  ('00)  reports  the  species  as  sporadic  in 
the  Elbe,  and  Skorikow  ('97)  finds  both  B.  bakeri  and  its  variety 
brevispinus  sparingly  in  the  Udy  in  summer  months. 

This  species  in  common  with  other  Brachionida:  was  infested  by 
Bimcerium  hyalinum  Przesm.,  and  occasionally  by  a  filamentous 
fungus-like  growth.  Empty  loricae  were  wont  to  appear  with  the 
culmination  of  a  pulse  and  subsequently.  No  males  were  identified 
as  belonging  to  this  species,  and  attached  male  eggs  were  recorded 
only  late  in  September,  1897,  at  the  close  of  an  unusual  pulse.  They 
were  found  on  var.  cluniorbicularis  and  rhenanus.  Females  with 
winter  eggs  were  not  at  any  time  recorded  for  this  species.  It  may 
be  that  some  of  the  free  winter  eggs  referred  to  the  genus  Brachionus 
(Table  I.)  belong  to  this  species.  The  recurrent  pulses  are  similar  to 
those  of  known  poly  cyclic  species,  and  we  may  infer  the  probability 
of  such  a  phenomenon  in  B.  bakeri,  though  conclusive  proof  of  its 
occurrence  is  not  found  in  the  statistical  records. 

Brachionus  budapestinensis  v.  Daday. — Average  number  of 
females,  4,211;  of  eggs  (carried),  740.  This  is  one  of  the  most 
sharply  defined  species  of  Brachionus  and  a  typical  planktont  of 
open  waters.  It  has,  moreover,  a  sharply  limited  seasonal  distribu- 
tion in  which  it  is  apparently  poly  cyclic.  The  appended  table  gives 
the  dates  and  temperatures  of  appearance  and  disappearance  and 
the  pulses  in  the  several  years. 

In  the  main,  the  period  of  occurrence  is  practically  from  the  end 
of  June  till  the  early  part  of  October  and  above  60°.  A  record  in 
May,  1896,  and  an  isolated  one  in  December  of  the  same  year,  indicate 
an  extension  of  this  period,  but  such  occurrences  are  rare  and 
irregular  and  the  numbers  small.  This  abrupt  decline  in  1898  as 
temperatures  pass  60°  (PI.  XII.,  Pt.  I.,  and  Table  I.)  is  paralleled 
in  previous  years.  The  normal  seasonal  routine  seems  to  be  as 
follows:  The  species  reappears  in  the  plankton  in  May- June  at 
70°,  rising  slowly  to  its  first  pulse  (average,  26,104)  in  July,  with  a 
larger  pulse  (average,  184,453)  in  the  following  month  during  the 
maximum  heat,  and  a  much  smaller  one  (average,  10,044)  in  Sep- 
tember, followed  immediately  by  an  abrupt  decline.  The  average 
temperature  of  the  larger  pulses  lies  close  to  the  season's  maximum, 
while  the  latest  pulse,  at  the  lower  temperature  (72.2°)  averages  but 
10,044.  These  data  all  indicate  that  this  is  a  midsummer  planktont, 
with  its  optimum  temperature  near  the  summer's  maximum.  The 


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177 

relation  of  hydrographic  conditions  to  the  relative  development  of 
pulses  in  different  years  is  seen  on  a  comparison  of  the  record  for 
1896  and  1897,  the  former  (Pt.  I.,  PI.  X.)  being  a  year  of  recurrent 
floods  and  the  latter  (Pt.  I.,  PI.  XI.)  one  of  stable  conditions 
through  the  greater  part  of  the  seasonal  distribution  of  the  species 
in  question.  The  average  numbers  in  these  two  years  were  3,105 
and  31,306,  respectively,  and  the  average  amplitude  of  the  pulses 
18,250  and  97,200,  showing,  respectively,  a  ten-  or  five-fold  increase 
in  the  latter  year.  The  extension  of  the  heated  termjnto  September 
in  1897,  is  reflected  in  the  large  September  pulse  (552,000)  and  in 
the  extension  of  the  period  of  occurrence  into  October. 

The  locations  of  the  pulses  of  Brachionus  budapestinensis  in  1898 
correspond  with  those  of  the  Ploima  in  general.  They  likewise 
coincide  with  or  follow  those  of  the  chlorophyll-bearing  organisms 
(cf.  PI.  I.  and  II.  with  III.  and  IV.  and  Table  I.).  Similar  relations 
are  apparent  in  1896  and  1897  but  are  less  evident  in  prior  years. 
They  suggest  an  interrelationship  of  the  pulses  in  this  species  with 
the  fluctuations  in  the  food  supply. 

Males,  male  eggs,  and  winter  eggs  were  not  recorded,  but  the 
recurrent  pulses  in  this  species  are  so  similar  to  those  in  other  rotifers 
in  which  the  evidence  of  the  occurrence  of  sexual  reproduction  at 
the  culmination  of  each  pulse  has  been  found,  that  the  inference 
may  be  made  that  this  species  likewise  is  poly  cyclic  in  our  waters. 
Females  carrying  one  or  two  summer  eggs  have  been  found  in 
greatest  abundance  during  the  rise  of  the  pulse,  and  only  in  small 
numbers,  if  at  all,  during  its  decline. 

This  species  is  subject  to  some  variation  in  the  development  of 
surface  ornamentation,  in  the  ratio  of  width  and  length,  and  in  the 
curvature  of  the  median  spines.  It  is  usually  somewhat  more 
slender  than  figured  originally  by  v.  Daday  ('85)  or  even  by  Hempel 
('96) ,  who  described  a  form  somewhat  more  slender  than  that  figured 
by  v.  Daday,  as  B.  punctatus.  Shortly  afterwards  Skorikow  ('96) 
described  the  same  species  as  B.  lineatus  from  Russian  waters.  The 
name  given  by  v.  Daday  has, priority,  and  as  neither  the  Russian  nor 
the  American  forms  are  to  my  mind  well  enough  set  off  to  merit 
even  varietal  distinction,  I  have  used  the  name  given  by  v.  Daday, 
and  have  included  under  it  both  wide  and  narrow  forms  and  those 
with  incurved  or  outcurved  median  spines.  The  fact  that  their 
common  record  of  seasonal  distribution  forms  a  seasonal  curve  of 


178 

typical  character  is  corroborative  of  the  view,  though  not  conclusive, 
that  we  are  dealing  with  a  single  species  and  not  with  several. 

This  species  has  not  been  widely  reported  in  the  fresh-water 
plankton.  It  is  evidently  a  planktont  of  warmer  waters,  and  for 
that  reason  may  have  escaped  notice,  since  the  cooler  waters  have 
been  the  more  thoroughly  explored.  Thus  it  was  not  found  by 
Weber  ('98)  in  Swiss  waters  in  his  thorough  explorations  about 
Geneva,  nor  by  Jennings  ('94,  '96,  '00)  in  the  Great  Lakes  or  inland 
waters  of  Michigan.  It  has,  however,  been  recorded  by  Skorikow 
('97)  in  the  plankton  of  the  Udy  River,  in  Russia,  where  it  was 
exceeded  in  number  by  only  two  species  of  its  genus,  B.  pala  and 
B.  angularis,  ranking  tenth  in  numbers  among  all  the  rotifers.  His 
data  of  frequency  from  July  to  October  suggest  several  recurrent 
pulses.  It  has  likewise  been  found  by  Lauterborn  ('98)  in  the 
plankton  of  the  Rhine,  where  he  classes  it  with  the  stenothermal 
planktonts.  Zacharias  ('98)  finds  it  in  ponds  near  Leipzig,  and  it 
was  originally  described  by  v.  Daday  ('85)  from  Hungarian  waters, 
and  again  noted  there  by  Kertesz  ('94).  Fuller  exploration  of  the 
summer  plankton  in  warmer  regions  will  doubtless  extend  the  record 
of  its  range. 

Brachionus  militaris  Ehrbg. — Average  number  of  females,  147 ; 
of  eggs  (carried),  98.  In  previous  years  the  species  was  much  more 
abundant,  the  averages  in  1897  being  1,412  females  and  523  eggs,  and 
in  1896,  1,288  females  and  576  eggs.  This  greater  development  in 
years  prior  to  1898  is  evident  in  many  of  the  Brachionidcz. 

The  following  table  gives  the  dates  of  first  and  last  records  in 
each  season,  and  the  location,  temperature,  and  amplitude  of  the 
pulses  in  the  several  years. 

This  is  evidently  a  summer  planktont  with  well-defined  limits. 
These  limits  appear  much  less  evident  in  1898  (Table  I.)  than  in 
prior  years.  In  1896  and  1897,  for  example,  the  species  is  almost 
continuously  present  in  the  plankton  from  the  time  of  its  first 
appearance  until  the  last  record  for  the  season.  All  of  the  records 
save  two  lie  above  70°,  and  the  average  temperatures  at  which  the 
pulses  occur  are  all  at  or  above  80°.  Its  optimum  thus  lies  near 
the  summer  maximum.  The  lower  limits  are  not  definitely 
established  owing  to  insufficient  collections  in  periods  of  rise 
and  decline,  but  they  seem  to  lie  near  70°,  with  small  numbers 
lingering  to  60°. 


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179 


180 

This  species  has  never  developed  large  pulses  in  the  channel 
waters  of  the  Illinois.  Hempel's  statement  ('99)  that  it  is  "the 
most  abundant  species  of  the  genus"  can  apply  only  to  certain 
collections  in  vegetation-rich  backwaters,  for  in  the  river  it  is  sur- 
passed in  the  totals  of  occurrences  in  the  statistical  records  by  eight 
other  forms  of  firachionus,  namely,  variabilis,  pala,  amphiceros, 
dorcas,  rubens,  budapestinensis,  duniorbicularis,  and  tuberculus.  I 
found  it  in  very  great  abundance  in  the  July-August  plankton  of 
Crystal  Lake,  a  shallow  warm  pond  rich  in  vegetation,  formed  by 
damming  a  small  creek  tributary  ta  the  Wabash  system,  near 
Urbana,  111.  From  the  relatively  small  numbers,  the  slight  ampli- 
tude of  the  pulses,  and  their  somewhat  irregular  development  I  am 
inclined  to  think  that  the  centers  of  distribution  of  this  species  are 
not  in  the  open  water  of  the  river  and  its  backwaters,  but  more 
in  the  vegetation  of  warm,  shallow  regions  such  as  the  margins  of  our 
bottom-land  lakes.  It  is  thus  to  some  extent  adventitious  in  our 
plankton. 

The  pulses  of  this  species  are  relatively  so  small  that  they  do  not 
contribute  an  appreciable  amount  to  the  total  ploiman  pulses,  nor 
do  more  than  50  per  cent,  of  their  number  coincide  with  such  general 
pulses,  though  they  are  sometimes  found  during  their  rise.  The 
greater  part  of  them  coincide  with  the  pulses  of  chlorophyll-bearing 
organisms  (PL  I.  and  II.),  suggesting  a  food  relationship. 

This  species  is  one  of  the  best-defined  in  the  genus,  though  in  the 
character  of  its  asymmetry  it  varies  toward  B.  bakeri  var.  tuberculus 
Turner.  It  exhibits  some  variation  in  the  degree  of  asymmetry,  in 
the  curvature  of  the  spines,  and  in  the  surface  markings.  The  indi- 
cations of  pulses  suggest  a  poly  cyclic  habit,  but  no  evidence  in  the 
way  of  males,  male  eggs,  or  winter  eggs  was  recorded  which  will 
substantiate  the  inference.  A  female  carrying  a  winter  egg  wras 
found  Sept.  21,  1-897,  at  the  close  of  the  period  of  occurrence.  Fe- 
males with  one,  two,  or  three  summer  eggs  were  found  throughout 
the  summer  and  in  somewhat  larger  numbers  during  the  rise  of  the 
pulses. 

Brachionus  mollis  Hempel. — Average  number  of  females,  137; 
of  eggs,  10.  More  abundant  in  previous  years,  the  average  in  1897 
being  1,092  and  277,  and  in  1896,  428  and  56. 

This  likewise  is  a  summer  planktont.  The  earliest  record  of  its 
appearance  in  the  plankton  is  June  17,  1896,  at  76°;  and  the  latest, 


181 


October  17,  1894,  at  58°.  With  but  two  exceptions  the  species  was 
taken  only  above  70°,  and  the  period  of  most  continuous  occurrence 
and  largest  numbers  is  near  the  summer  maximum  of  80°.  The 
optimum  is  thus  near  the  summer  maximum.  This  species  was 
never  taken  in  the  plankton  in  large  numbers,  the  greatest  being  on 
Sept.  14,  1897  (20,000),  at  84°.  On  account  of  the  small  numbers 
.and  somewhat  irregular  occurrences  the  phenomenon  of  recurrent 
pulses  is  here  less  apparent  than  it  is  in  more  abundant  species.  The 
appended  table  records  the  best-defined  ones.  Thesejpulses  share  in 
the  general  ploiman  pulses  in  only  about  50  per  cent,  of  the  cases, 
and  the  most  of  them  coincide  with  or  follow  shortly  after  the  pulses 
of  chlorophyll-bearing  organisms. 

PULSES  OF  BRACHIONUS  MOLLIS 


Year 

Date 

Temp. 

No. 

Date               Temp. 

No. 

1895  

July    6 

81° 

742 

Sept.     5                75° 

954 

1896  

July  18 

79° 

1,200 

Aug.     21    \           79° 

8  ,  400 

1897  

July  30 

85° 

1  1  ,  600 

Sept.     7                80° 

20,000 

1898  

Aug.  23 

81° 

800 

Sept.  27                73° 

4,800 

So  far  as  I  am  aware  this  species  has  not  been  found  in  other 
waters  than  the  Illinois  River  and  its  adjacent  backwaters.  Hempel 
('99)  reports  it  as  most  abundant  in  the  marshy  environment  of 
Flag  Lake. 

Brachionus  pala  Ehrbg. — Average  number,  including  all  varie- 
ties: females,  19,969;  eggs,  25,974.  The  following  table  gives  the 
average  number,  in  the  several  years,  of  the  varieties  here  included, 
and  it  will  serve  to  show  their  relative  frequency. 

This  is  the  most  abundant  species  of  the  genus  in  our  waters,  the 
grand  total  of  all  occurrences  exceeding  9,000,000.  As  a  whole  the 
species  was  much  more  abundant  in  the  stable  year  1897  (180,998), 
and  less  abundant,  all  things  considered,  in  the  disturbed  conditions 
of  1896  (36,665).  As  a  whole  the  type  form  pala  is  less  abundant 
than  amphiceros.  It  forms  but  28  per  cent,  of  the  total,  as  compared 
with  68  per  cent,  included  in  the  latter  variety.  Dorcas  forms  less 


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183 

than  2  per  cent ;  and  the  form  spinosus,  less  than  1  per  cent.  The 
proportions  formed  by  the  several  varieties  fluctuate  from  year  to 
year  and  from  season  to  season, — indeed,  from  collection  to  collec- 
tion (Table  I.).  Thus  in  the  first  three  years  pala  exceeded  am- 
phiceros,  while  in  the  last  two  these  conditions  were  reversed; 
and  in  1896  the  form  spinosus  contributes  6.5  per  cent,  of  the 
individuals.  The  predominance  of  the  pala-amphiceros  group  is, 
however,  preserved  throughout  all  of  the  years. 

The  species  as  a  whole  is  found  throughout  the  -entire  seasonal 
range  of  temperatures  but  with  very  great  fluctuations  in  numbers. 
Speaking  generally,  there  are  vernal  and  autumnal  pulses  separated 
by  a  midwinter  minimum  which  is  well  sustained,  developments  in 
excess  of.  5 ,000  per  m3  being  very  rare  in  this  season.  There  is  also 
a  midsummer  minimum  more  or  less  diversified  by  pulses  of  some 
magnitude.  This  sequence  was  not  fully  realized  in  any  single  year 
of  our  records,  but  this  may  be  due  in  part  to  insufficient  collections 
at  times  of  the  major  pulses.  Thus  in  1894  only  a  small  autumnal 
pulse  (13,650)  was  detected.  In  1895,  there  was  a  small  vernal 
pulse  (67,338),  and  a  belated  autumnal  pulse  (320,915)  lasting  a  full 
month  in  November-December.  In  1896,  there  was  a  very  abrupt 
vernal  pulse  rising  from  53,618  on  April  17  to  1,012,350  on  April  24, 
while  in  the  fortnightly  fall  collections  the  only  pulse  detected  was 
one  of  14,000.  In  1897,  the  monthly  collections  of  the  spring  seem 
to  have  missed  all  considerable  developments,  the  largest  recorded 
being  only  16,000.  On  August  31  and  October  12  of  that  year, 
however,  there  were  pulses  of  1,398,000  and  1,605,600.  In  1898 
there  was  a  well-developed  vernal  pulse  of  451,200  and  a  small 
autumnal  one  of  83,200. 

The  species  is  not,  however,  dicyclic,  for  both  the  winter  and 
summer  interims  are  marked  by  occasional  recurrent  pulses  of 
smaller  proportions.  The  table  on  the  next  page  shows  the  loca- 
tions and  temperatures  of  the  culminations  of  these  pulses. 

From  this  table  it  is  evident  that  a  wide  range  of  optimum  tem- 
peratures is  possible.  Nevertheless,  23  of  the  31  pulses  occur  above 
50°,  and  21  of  them  above  60°.  In  1898  only  3  per  cent,  of  the 
individuals  are  found  below  57°,  and  with  the  exception  of  1895 
approximately  these  conditions  will  be  found  in  the  other  years. 
Brachionus  pala  is  thus  a  perennial  planktont,  but  as  a  rule  it  reaches 
its  largest  developments  only  above  60°  in  our  channel  waters. 


184 


PULSES  OF  BRACHIONUS  PALA  AND  VARIETIES. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1896 

Tan      1 

33° 

8   268 

1897 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Apr   24 

79° 

1   012   350 

7  ^° 

„ 

Year 

Date 

Temp. 

No.  . 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 

July  30 
July    6 

82° 
81° 

1,908 
3,710 

Sept.    4 
Sept.  20 

78° 
79° 

13,650 
2,223 

Aug.  21 

81° 

47,480 

1896 

1897 
1898 

July  23 

July  30 
July  19 

78° 

84° 
84° 

12,600 

11,200 
6,400 

Aug.    3 
"      15 

Aug.  31 
Aug.  16 

80° 
81° 

80° 

77° 

39,200 
12,800 

1,398,000 
38,400 

Sept.  30 

58° 

14,000 

Sept.  27 

73° 

83,200 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 

1895 

1897 

Oct    12 

65° 

1   605   600 

1898 

Oct.  25 

49° 

8,500 

Nov.  15 

41° 

1,100 

Dec.  15 

32° 

3,100 

185 

The  pulses  recorded  in  the  table  will  be  found  to  coincide  (Table 
I.)  with  those  of  other  species  of  the  genus,  and  in  the  main  with 
those  of  the  total  Ploima,  thus  indicating  that  this  species  responds, 
along  with  other  rotifers,  to  some  common  factor  of  their  environ- 
ment. The  relation  of  these  pulses  to  those  of  the  chlorophyll- 
bearing  organisms  (PI.  I.  and  II.)  is  also  striking.  Of  the  30  pulses 
recorded  in  the  table,  6  fall  outside  of  the  period  included  in  Plates 
I.  and  II.  Of  the  remaining  24  there  are  17  whose  culminations  in 
the  main  coincide  with  those  of  the  organisms  upon  which  they  feed, 
and  5  of  the  6  remaining  follow  shortly  thereafter,  usually  at  the 
next  collection,  at  an  interval  of  a  week  or  thereabouts.  In  one 
case  only  is  there  a  delay  of  a  fortnight  after  all  of  the  plant  pulses. 
The  large  pulses  of  August-October,  1897,  were  judged  by  the 
ChlorophycecB  only,  as  these  overtop  the  other  plants  so  greatly.  The 
pulse  of  August  31  occurs  a  week  before  the  culmination  of  the 
Chlorophycecz  is  reached,  but  in  the  presence  of  abundant  food.  The 
dependence  of  these  pulses  of  Brachionus  pala  upon  the  food  supply 
is  plainly  suggested  by  their  time  relations  with  the  pulses  in  the 
-plant  life  of  the  plankton. 

Further  reason  for  concluding  that  the  species  is  polycyclic  is 
found  in  the  evidences  of  sexual  reproduction,  which  will  be  noted 
in  connection  with  the  discussion  of  the  varieties.  In  this  connec- 
tion it  will  suffice  to  say  that  there  is  some  evidence  that  the  pulses 
are  preceded  by  rapid  parthenogenetic  reproduction,  and  accom- 
panied or  followed  by  the  appearance  of  male  eggs,  males,  and 
winter  eggs. 

The  eggs  of  Brachionus  pala  are  detached  from  the  parent  in 
such  a  large  proportion  of  the  cases  in  preserved  material  that  the 
tracing  of  the  reproductive  cycle  by  means  of  attached  eggs  is  ren- 
dered difficult  if  not  impossible.  Furthermore,  eggs  resembling 
the  winter  eggs  of  this  species,  and  provisionally  referred  to  it  in  our 
records,  are  to  be  found  in  the  plankton  at  nearly  all  seasons  of  the 
year,  and  it  is  obviously  impossible  to  determine  the  time  at  which 
they  were  produced.  It  seems  probable  that  all  of  the  varieties 
pass  through  recurrent  cycles,  and  that  none  of  them  is  a  temporary 
phase  of  the  cycle. 

Outbreaks  of  parasitic  diseases  in  this  species  are  very  common. 
They  almost  always  attend  the  larger  pulses,  but  isolated  individuals 
infested  by  some  of  these  pests  are  not  infrequent,  especially  during 


186 

the  summer  months.  Thus  in  the  vernal  pulse  of  pala  (type  only) 
reaching  716,982  on  April  24,  1896,  19,056  individuals  were  para- 
sitized by  Bim&rium  hyalinum  Przesm.,  or  by  something  very 
similar  to  it,  and  30,966  were  infested  by  a  fungus-like  growth.  This 
is  about  7  per  cent,  of  the  total  individuals.  Similar  though  less 
pronounced  outbreaks  have  -  attended  other  vernal  and  autumnal 
pulses.  Species  of  Colacium  are  sometimes  found  attached  to  the 
loricas  of  this  s*pecies. 

Brachionus  pala  is  exceedingly  variable,  especially  in  the  matter 
of  the  development  of  the  posterior  spines.  Forms  without  the 
spines  (pala  type)  intergrade,  by  only  slight  gradations,  into  those 
with  fully  developed  spines  (var.  amphiceros).  The  angle  which 
these  spines  make  with  the  lorica  is  also  a  matter  of  great  variation, 
in  preserved  material  at  least.  Individuals  with  the  spines  at  right 
angles  to  the  antero-posterior  axis  are  occasionally  seen.  The 
species  also  varies  in  the  matter  of  the  dorsal- ventral  curvature  of 
the  antero-median  spines  (var.  dorcas}.  Individuals  with  such 
curved  antlers  are  sometimes  provided  with  posterior  spines  (var. 
dorcas  form  spinosus}.  I  have  followed  Weber  ('98)  in  placing 
B.  amphiceros  Ehrbg.,  B.  dorcas  Gosse,  and  its  form  spinosus  Wierz. 
as  varieties  of  B.  pala.  They  do  not,  however,  all  stand  upon  an 
equal  footing.  B.  amphiceros  grades  imperceptibly  into  B.  pala,  and 
has  the  same  seasonal  distribution.  B.  dorcas  and  its  form 
spinosus  intergrade  with  each  other  as  do  pala  and  amphiceros,  and 
they  also  exhibit  some  intergradations  with  B.  pala;  but  they  are 
winter  varieties,  or  at  least  belong  to  the  colder  season,  as  will 
appear  later.  Their  differentiation  in  this  respect  is  thus  more 
striking  than  that  of  B.  amphiceros,  and  makes  it  probable  that  we 
have  in  dorcas  a  seasonal  variety  of  B.  pala.  Zacharias  ('98)  has 
reduced  B.  pala  to  a  variety  of  B.  amphiceros  because  in  his  opinion 
the  latter  is  the  more  widely  distributed  form  in  certain  pond  waters 
which  he  examined.  This  is  a  criterion  which  presupposes  a  wide 
knowledge  of  distribution  and  numbers,  and,  furthermore,  a  basis 
which  can  not  fail  to  add  to  the  confusion  already  existing  in  this 
genus,  since  it  is  hardly  to  be  hoped  that  it  will  lead  to  the  same 
conclusion  in  the  hands  of  different  investigators  in  different  regions, 
or  even  in  different  seasons  and  years  in  the  same  region.  As  an 
illustration  of  the  difficulties  which  might  arise  I  may  cite  the  yearly 
averages  of  amphiceros  and  pala  in  the  table  on  page  182.  In  three 


187 

years  the  latter  is  more  abundant,  and  in  two,  the  former.  The 
relative  abundance  of  these  forms  in  the  river  at  a  given  point  of 
collection  is  an  epitome  of  their  distribution  in  a  wide  area  of  channel 
and  backwaters.  An  application  of  the  principle  advanced  by 
Zacharias  would  in  this  instance  lead  to  constant  change.  The 
retention  of  pala  (Ehrbg.,  1830)  as  the  type  and  amphiceros  (Ehrbg., 
1838)  as  the  variety  is  in  keeping  with  priority  in  nomenclature  and 
with  the  principle  of  regarding  the  more  highly  differentiated  or 
divergent  form  as  the  variety.  Variety  amphiceros  occupies  thus 
the  same  relation  to  the  type  that  bidens  does  to  its  type  angularis. 
Both  are  illustrations  of  the  tendency  common  to  all  species  of 
Brachionus  to  develop  posteriorly  directed  spines. 

I  shall  proceed  to  discuss  the  salient  points  in  the  seasonal  dis- 
tribution and  statistics  of  the  several  varieties : — 

Brachionus  pala  Ehrbg.,  type. — Average  number  of  individuals, 
2,693  ;  of  eggs,  20,809,  including  all  free  eggs  referable  to  the  species 
in  the  broader  sense.  In  the  present  connection  I  shall  call  attention 
only  to  the  fact  that  the  type  form,without  the  posterior  spines,  is  less 
abundant  during  the  midsummer  interval  than  the  spinous  variety 
amphiceros.  This  appears  in  Table  I.,  and  is  to  be  found  in  the 
records  of  years  prior  to  1898.  A  fuller  comparison  of  the  records 
of  the  two  forms  will  be  made  in  the  discussion  of  amphiceros.  I 
shall  not  discuss  the  recurrent  pulses  of  this  form  or  of  amphiceros, 
since  as  they  dominate  those  of  the  species  as  a  whole  it  would  lead 
to  considerable  repetition.  The  pulses  of  pala  in  the  main  (Table  I.) 
coincide  in  location  with  those  of  the  species  as  a  whole,  and  the 
direction  of  movement  of  the  seasonal  curve  of  distribution  is  quite 
similar,  save  in  the  fact  that  the  amplitude  of  the  pulses  is  less,  and 
that  the  differences  in  seasonal  distribution  between  pala  and 
amphiceros  modify  the  curve  of  each. 

The  decisive  evidence  of  sexual  reproduction  in  the  species  in  the 
form  of  attached  male  and  winter  eggs  is  found  repeatedly  at  times 
of  the  major  pulses.  In  some  instances  they  appear  during  the  rise 
of  the  pulse.  The  autumnal  pulse  of  1895  will  serve  as  an  illustra- 
tion of  the  character  of  these  statistical  data.  (See  following  page.) 

This  pulse  is  sustained  much  longer  than  usual,  but  it  serves  to 
show  the  prevalence  of  parthenogenetic  eggs  during  the  rise  of  the 
pulse,  and  the  evidence  of  sexual  reproduction  during  its  progress. 
In  some  other  instances  the  number  of  free  winter  eggs  after  the 


188 


BRACHIONUS  PALA,  TYPE  FORM.     SEXUAL  CYCLE. 


1895 

Males 

Male  eggs 
carried 

Winter  eggs 

Summer  eggs 

Total 
eggs* 

Total 
indi- 
vidxials: 

Free* 

Car- 
ried 

Free* 

Carried 

n/M-     30 

96 
1,700 
63,135 
43,680 
46,746 
68,310 
16,112 
17,808- 

96 
6,375 
150,075 
114,240 
138,754 
135,930 
22,472 
18,921 
742 

4S 
6,885 
134,550 

189,280' 
j 

217,777 
211,830 
36,464 
14,469- 
371 

Nov     5  

4,140 

765 
8,280 
1,680 
742 
1,380 
424 

765 
7,245 

85 

3,060 
71,415 
68,880  . 
89,040 
66,240 
17,384 
1,113 
371 

Nov.  14  

Nnv     90 

Nov.  27  



2,226 

— 

F)pr-         4- 

DPP      1  1 

F)pr       1  8 



ri<=>p      9  ^ 

*  Includes  free  eggs  of  other  varieties  also. 

culmination  of  a  pulse  is  very  large.  For  example,  the  sudden  ver- 
nal pulse  of  716,982  on  April  24  is  accompanied  by  28,584  free 
winter  eggs.  The  pulse  declines  to  22,224  on  April  29,  and  the  free 
winter  eggs  rise  to  95,841,  and  the  empty  loricas  to  26,114. 

Females  carry  1-5  summer  eggs,  and  1-8,  or  even  more,  male 
eggs.  There  is  great  variation  in  the  size  of  the  summer  eggs,  these 
and  -the  male  eggs  appearing  almost  to  intergrade. 

Brachionus  pala,  including  B.  amphiceros,  is  a  common  constitu- 
ent of  the  plankton  of  shallow  warm  waters.  It  has  not  been 
reported  from  the  larger  and  cooler  lake  waters  by  Apstein  ('96), 
Burckhardt  ('00  and  'OOa),  or  Jennings  ('94,  '96,  and  '00).  Zacha- 
rias  ('98)  and  Marsson  ('00)  find  it  in  the  summer  plankton  of 
smaller  lakes  and  ponds  in  Germany.  Seligo  ( '00)  records  it  from 
April  to  October,  with  a  maximum  in  August,  in  Prussian  lakes ;  and 
Lauterborn  ( '98a)  finds  it  to  be  perennial  and  polycyclic  in  -the 
Rhine.  Schorler  ('00)  reports  both  pala  and  amphiceros  from  the 
Elbe,  the  former  being  abundant  in  May  and  sporadic  during  the 
summer,  while  the  latter  was  abundant  in  April,  June,  and  Septem- 
ber, and  rare  at  other  times  during  the  warmer  months.  Zimmer 


189 


('99)  finds  amphiceros  in  the  Oder,  where  it  appears  in  April  and. 
increases  until  the  end  of  August  or  the  first  of  September,  when 
it  is  the  most  abundant  animal  in  the  plankton.  In  no  one  of  these 
instances  was  the  examination  so  long  continued  or  made  at  such 
short  intervals  as  in  the  case  of  the  exploration  of  the  Illinois.  The 
diversity  exhibited  in  these  different  waters  may  be  paralleled  by 
the  fluctuations  from  year  to  year  in  the  Illinois,  and  from  all  the 
data  it  may  be  inferred  that  the  organism  is  probably  perennial  and 
polycyclic,  the  number  of  pulses  depending  upon  local  conditions, 
primarily  of  the  food  supply. 

Brachionus  pala  var.  amphiceros  Ehrbg. — Average  number  of 
females,  17,071;  of  eggs,  5,103.  The  numbers  were  much  larger 
(158,299  and  35,392)  in  the  stable  conditions  of  1897,  and  still 
smaller  (5,430  and  715)  in  the  disturbed  conditions  of  1896. 

The  seasonal  distribution  of  this  variety  with  respect  to  that  of 
the  type  constitutes  the  chief  point  of  interest  in  the  records.  It  is 
present  throughout  the  whole  range  of  temperatures,  shares  in  the 
vernal  and  autumnal  pulses  noted  for  the  species  as  a  whole,  but 
constitutes  a  much  greater  proportion  of  the  amphiceros-pala  group 
during  the  warmer  months  than  it  does  in  the  colder  ones.  Thus, 
as  shown  in  the  accompanying  table,  the  proportion  which  amphi- 

SEASONAL  DISTRIBUTION  OF  BRACHIONUS  PALA    AND    B.  PALA  VAR.  AMPHICEROS. 


Year 

June  1  to  Oct.  1 

Oct.  1  to  June  1 

pala 

amphiceros 

pala 

amphiceros 

No. 

\->  j-> 

£  G 

PH   g 

No. 

t-i   -4J 

&     C 

PH  g 

No. 

IH   w 

cu  c 
PH  g 

No. 

t-c  -»-> 

<u  ej 

PnO 

1895  

7,042 
14,637 
14,600 
10,440 

4 
14 
4 
4 

155,324 
89,400 
3,776,400 
229,720 

96 
86 
96 
96 

863,247 
958,265 
719,650 
129,623 

71 
87 
44 
17 

336,618 
144,087 
912,580 
657,960 

29 
13 
66 
83 

1896  

1897    

1898  

Average  

11,679 

6.5 

1,062,711 

93.5 

667,696 

55 

512,811 

45 

190 

ceros  forms  of  this  group  in  the  period  from  June  1  to  October  1  is 
from  86  to  96  per  cent.,  averaging  93.5  per  cent,  in  the  several  years. 
On  the  other  hand,  in  the  colder  months — Jan.  1  to  June  1  and  Oct.  1 
to  June  1 — the  per  cent,  is  only  from  13  to  83,  averaging  45.  The 
temperatures  on  June  1  (Pt.  I.,  PL  IX. -XII.)  average  about  75°, 
and  on  Oct.  1  about  67°.  The  spinous  form  (amphiceros)  thus  in- 
cludes about  45  per  cent,  of  the  individuals  at  low  temperatures,  and 
93.5  per  cent,  at  high  temperatures;  and  the  smoother  form  (pala 
type),  55  per  cent,  and  6.5  per  cent.,  respectively. 

This  predominance  of  the  spinous  variety  at  high  temperatures 
is  apparently  a  striking  illustration  from  statistical  evidence  of  the 
hypothesis  of  Wesenberg-Lund  ('00)  that  such  elongations  of  the 
body  of  planktonts  are  adaptations  to  the  lessened  buoyancy  of  the 
warmer  water.  This  relation  of  the  spinous  form  to  higher  tem- 
peratures is  evident  in  every  year,  1895-1898,  and  the  proportion 
of  spinous  forms,  86-96  per  cent.,  exhibits  all  the  constancy  that 
might  at  the  best  be  expected  in  plankton  data.  The  relation  is 
generally  apparent  (Table  I.)  in  the  individual  entries  as  well  as  in 
the  sums  total,  and,  considering  the  numbers  concerned  and  the 
long  period  of  observation,  should  have  more  weight  than  some  of 
the  exceptions  to  the  hypothesis,  which  have  been  or  will  be  noted, 
in  which  the  data  are  less  extensive.  For  example,  Brachionus 
pala  var.  dorcas  does  not  in  its  seasonal  distribution  support  the 
hypothesis,  but  owing  to  its  small  numbers — especially  of  the  form 
spinosus — less  weight  should  attach  to  its  evidence. 

In  1897  the  first  autumnal  pulse  of  the  pala  group  consisted 
almost  entirely  of  var.  amphiceros.  This  pulse  started  August  10 
at  3,600,  culminated  August  31  at  1,398,000,  and  declined  to  800 
September  29.  Of  the  3,500,200  individuals  included  in  this  pulse, 
all  but  1 1 ,400  belonged  to  amphiceros.  The  temperatures  recorded 
during  this  period  ranged  from  8-3°  to  71°.  A  second  pulse  started 
October  5  at  1,600,  culminated  October  12  at  1,605,600,  and  declined 
to  0  on  October  26.  Of  the  total  individuals  (1,609,000)  included  in 
this  pulse,  894,800  belonged  to  amphiceros  and  7 14,200  to  pala.  The 
range  in  recorded  temperatures  in  this  period  was  from  71°  to  59.5°. 
This  may  serve  as  an  additional  illustration  of  the  relation  of  tem- 
perature to  the  spinous  variety  of  Brachionus  pala. 

This  variety  is  itself  poly  cyclic,  as  is  evidenced  by  the  recurrence 
of  male  and  winter  eggs  carried  by  the  female  at  times  of  the  pulses. 


191 


Owing  to  the  ease  with  which  such  eggs  are  detached,  the  records 
are  quite  imperfect  indices  of  the  actual  numbers.  In  1898  male 
eggs  (carried)  to  the  number  of  70,400  per  m.3  attended  the  culmina- 
tion of  the  vernal  pulse  (419,200)  on  May  3.  Winter  eggs  (carried) 
were  recorded  twice  on  the  decline  of  the  pulse  of  August  1 6 ;  once 
on  the  decline  of  that  of  October  2  5  ;  and  once  on  that  of  December  1 5 . 
Brachionus  pala  var.  dorcas  Gosse. — The  seasonal  distribution 
of  this  variety  is  so  sharply  defined  that  it  merits  especial  attention. 
The  following  table  gives  the  dates  and  temperatures  of  last  and 
first  records  in  each  year. 

SEASONAL  LIMITS  OF  BRACHIONUS  PALA  VAR.  DORCAS. 


Year 

Last  records 

First  records 

Largest  pulses 

Date 

Temp. 

Date 

Temp. 

Date 

Temp. 

No. 

1895 

Apr.  29 

64° 

Oct.    15 
Nov.  14 

57° 
46° 

Apr.  29 

64° 

9,000 

1896 

May     1 

70° 

Nov.  17 

44° 

Apr.  24 

72° 

183,000 

1897 

Apr.  27 

60° 

Oct.   12 
Jan.    11,  '98 

65° 
32° 

Apr.  27 

60° 

2,400 

1898 

Apr.  26 
May  17 

57° 
64° 

Dec.     6 

34° 

Apr.  26 

57° 

4,000 

The  species  practically  disappears  at  the  end  of  April,  when 
temperatures  rise  above  70°,  and  it  does  not  return  to  the  plankton 
until  they  fall,  in  October  and  November.  Its  period  of  continuous 
occurrence  does  not  begin  in  years  of  greatest  numbers  until  tem- 
peratures reach  45°,  and  it  remains  throughout  the  period  of  mini- 
mum temperatures.  As  the  collection-averages  indicate,  this 
species  is  relatively  rare,  and  its  numbers,  even  in  its  largest  pulses, 
are  usually  smaller  than  those  of  the  other  varieties  which  it  accom- 
panies. Although  this  species  is  a  winter  planktont  it  reaches  its 
greatest  development  during  the  spring  pulse,  indicating  an  opti- 
mum near  65°,  though  it  does  not  recur  in  numbers  when  this 
temperature  returns  in  autumn.  There  is  a  single  autumnal  pulse 
in  1895  of  8,625,  on  November  14,  at  44°,  accompanying  pulses  in 
the  other  varieties.  There  was  also  one  midsummer  record. 


192 

The  curvature  of  the  median  anterior  horns  which  defines  this 
variety  results  in  a  considerable  elongation  of  these  processes.  With 
regard  to  the  idea  of  Wesenberg-Lund  ('00)  that  this  tendency  on 
the  part  of  plankton  organisms  to  elongate  in  "  Balanceapparat "  is 
an  adaptation  to  the  lessened  buoyancy  of  the  warmer  water  of 
summer,  it  must  be  said  that  it  seems  difficult  to  apply  this  hypothesis 
in  the  case  of  B.  pala  var.  dorcas,  which  is  probably  a  seasonal  variety 
confined  to  winter  months.  I  have  no  data,  however,  on  the  relative 
development  of  these  processes  in  B.  pala  at  different  temperatures, 
beyond  the  seasonal  limitation  of  this  variety  to  lower  temperatures 
when  it  should  be  least  expected  according  to  the  hypothesis. 

Brachionus  pala  var.  dorcas  has  not  been  found  widely  distributed 
in  the  fresh- water  plankton,  or  at  least  not  reported  separately 
from  B.  pala,  which  is  widely  distributed.  Skorikow  ( '96)  reports 
it  from  Charkow,  Russia;  and  Kertesz  ('94),  in  January  from 
Budapest. 

Brachionus  pala  var.  dorcas  forma  spinosus  Wierz. — Average 
number  of  females,  33  ;  of  eggs,  2.  This  form  was  always  sporadic 
in  its  appearance  in  our  plankton.  Of  12  occurrences,  3  were  in 
April,  2  each  in  November,  December,  and  July,  and  one  each  in 
January,  May,  and  August.  The  whole  seasonal  range  of  tempera- 
tures is  thus  included.  It  may  be  of  significance  for  Wesenberg- 
Lund 's  hypothesis  that  the  spinous  form  of  dorcas  makes  over  50 
per  cent,  of  its  appearances  between  April  1  and  September  30, 
whereas  dorcas  itself  is  much  less  abundant  relatively  within  these 
limits.  The  largest  occurrence  of  spinosus — 100,044  on  April  24, 
1896 — wras  marked  by  the  fact  that  97.5  per  cent,  of  the  individuals 
were  infested  with  fungi.  The  nearest  approaches  to  pulses  in  this 
form  are  the  November-December  appearances  in  1895  and  1896. 
Females  with  winter  eggs  were  recorded  December  29  in  the  latter 
year. 

Brachionus  quadratus  Rousselet. — Individuals  corresponding  to 
Rousselet  's  description  have  been  found  occasionally  in  the  plankton 
from  the  last  of  May  till  the  middle  of  August  at  temperatures  of 
70°  and  above.  The  species  is  somewhat  closely  related  to  the 
bakeri  series,  and  may  ultimately  prove  to  belong  to  it.  Rousselet 
('97)  is  of  the  opinion  that  it  is  distinct  by  reason  of  the  truncate 
posterior  end,  the  absence  of  foot  sheath,  the  reticulations  of  the 
..shell,  and  the  semi- jointed  foot.  It  occurred  only  in  small  numbers, 


193 


and  forms  intermediate  between  it  and  bakeri  were  not  recorded. 
This  is,  I  believe,  the  first  record  of  its  occurrence  in  American  waters. 
Brachionus  urceolaris  Ehrbg. — Average  number  of  individuals 
including  all  varieties,  468;  of  eggs,  56.  The  species  was  relatively 
quite  abundant  in  1897  (5,290  and  1,976)  in  the  stable  conditions 
then  prevailing,  but  less  so  in  the  recurrent  floods  of  1896  (1,020 
.and  494).  It  is  not  a  common  species,  being  outranked  by  B. 
angularis,  bakeri,  budapestinensis,  and  pala.  The  species  as  a 
whole  is  found  throughout  the  entire  year,  though__never  in  large 
numbers  since  1895.  The  following  table,  which  gives  the  principal 
pulses  in  the  several  years,  shows  the  wide  range  of  the  species  and 
its  varieties  in  seasonal  distribution. 

PULSES  OF  BRACHIONUS  URCEOLARIS. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 

1895 

1896    .         ...           

Mar.  24 

41° 

2,727 

Apr.    17 

66° 

8,398 

1897 

Apr    27 

60° 

6,400 

1898    

Mar.  22 

51° 

2,000 

Apr.   26 

57° 

6,400 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

- 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

345 

Dec      3 

32° 

794 

Dec     13 

33° 

500 

194 

There  is  some  tendency,  especially  in  later  years,  toward  the 
colder  months.  Eight  of  the  fifteen  pulses  occur  below  70°,  and 
twelve  between  September  1  and  May  1 . 

On  account  of  the  small  numbers  the  pulses  are  poorly  defined 
in  our  records  (Table  I.),  but  there  are  indications  that  they  coincide 
in  location,  in  a  general  way,  with  those  of  other  Brachionid<z  and 
the  Ploima  as  a  whole.  They  also  in  many  instances  coincide  with 
or  follow  shortly  after  the  pulses  of  chlorophyll-bearing  organisms, 
as  has  been  noted  in  other  Brachionida. 

This  species,  B.  urceolaris,  is  a  cosmopolite,  and  of  general  occur- 
rence in  the  fresh-water  plankton  of  smaller  and  warmer  bodies  of 
water.  It  is  reported  by  Weber  ('98)  from  Swiss  marshes,  by 
Zacharias  ('98)  and  Marsson  ('00)  from  many  smaller  German 
waters,  and  by  Seligo  ('00),  throughout  the  year,  from  lakes  near 
Danzig,  where  it  attains  maxima  in  April,  July,  and  September. 
Since  this  author  includes  B.  angularis  (B.  urceolaris  forma  angu- 
latus  Seligo)  with  his  records  of  urceolaris,  it  is  probable  that  the 
species  in  the  usual  sense  may  have  much  more  restricted  numbers 
and  range  in  his  region.  Kertesz  ('94)  finds  it  about  Budapest. 
It  is  reported  as  sporadic  in  the  vernal  plankton  of  the  Elbe  by 
Schorler  ('00),  and  is  listed  from  the  Oder  by  Zimmer  ('99). 
Skorikow  ('97)  reports  it  once  in  summer  plankton  of  the  Udy 
near  Charkow. 

The  species  is  exceedingly  variable  in  the  development  of  the 
anterior  spines,  and  in  the  proportions  of  the  body.  It  varies 
toward  the  bakeri  group,  and  individuals  are  sometimes  found  which 
seem  to  connect  the  two  groups.  I  follow  Skorikow  ( '96)  in  placing 
B.  rubens  as  a  variety  of  B.  urceolaris,  including  in  it  those  forms 
whose  anterior  spines  are  least  developed.  The  more  slender 
summer  forms  I  have  listed  as  var.  bursarius  Barrois  and  v.  Daday. 
From  my  observations  on  B.  variabilis  Hempel,  I  am  inclined  to 
regard  it  as  a  possible  variety  in  the  urceolaris  group.  In  form, 
texture,  proportions,  and  anterior  spines  it  is  certainly  similar  to 
this  group.  The  presence  of  the  posterior  spines  would  not  suffice 
to  separate  it,  since  these  may  or  may  not  be  present,  and  the 
existence  of  a  variety  of  urceolaris  with  such  spines  would  only 
present  a  phenomenon  parallel  to  that  observed  in  pala,  angularis, 
and  bakeri.  The  quadrate  foot-plate  present  in  variabilis,  which, 
according  to  Hempel  ('96),  is  not  found  in  other  species  of  the 


195 

genus,  serves  to  distinguish  this  form,  and  in  the  absence  of  proof 
of  its  occurrence  in  jbrms  of  urceolaris  as  here  defined  I  prefer 
to  leave  variabilis  as  a  separate  species.  In  any  event  it  is  closely 
related  to  the  urceolaris  group,  and  may  ultimately  be  found  to 
belong  within  its  seasonal  range  of  variation.  Seligo  ('00)  has 
suggested  that  B.  angularis  is  also  a  variety  of  urceolaris,  but  I  do 
not  so  regard  it.  The  averages  of  the  different  forms  in  the  several 
years  are  given  in  the  table  on  the  next  page,  which  also  includes  B. 
variabilis.  The  discussion  of  the  different  varieties  follows : — 

Brachionus  urceolaris  Ehrbg.,  type. — Average  number  of  in- 
dividuals, 18.  The  type  form  was  not  abundant  in  any  year,  and 
its  appearances  were  sporadic.  It  was  recorded  in  February,  June, 
and  July.  It  includes  less  than  one  per  cent,  of  the  individuals 
referred  to  this  species. 

Brachionus  urceolaris  var.  rubens  Ehrbg. — Average  number  of 
individuals,  244;  of.  eggs,  41.  This  variety  was  more  abundant 
during  the  stable  conditions  of  1897  (5,290  and  1,976)  and  the  low- 
water  years  of  1894  and  1895.  It  includes  over  99  per  cent,  of  all 
the  individuals  referred  to  this  species. 

It  is  apparently  the  winter  form  of  the  species.  This  appears 
clearly  in  its  seasonal  distribution  in  the  later  years,  but  in  1894 
and  1895  it  was  found  in  summer  months  and  in  large  num- 
bers. It  is  thus  capable  of  development  in  the  whole  range  of 
temperatures. 

The  pulses  recorded  in  the  table  on  page  193  are  in  the  main 
composed  of  this  variety.  It  is  quite  abundant  during  the  summer 
of  1894,  attaining  a  pulse  of  181,764  on  August  15  at  84°,  disappear- 
ing in  September,  and  not  reappearing  until  the  April  collection.  It 
attains  a  pulse  of  324,254  on  June  19  at  80°,  declines  in  July,  then 
occurs  sporadically  until  the  following  February.  It  then  continues 
till  June  6,  with  a  pulse  of  8,398  on  April  17  at  66°.  An  isolated 
occurrence  of  10,000  in  July  is  the  only  record  in  the  summer  of 
1896.  It  is  in  the  November-December  plankton  of  1896  and  the 
March-May  plankton  of  1897,  and  attains  a  pulse  of  only  6,400  on 
April  2  7 ,  at  60°.  It  does  not  reappear  until  the  14th  of  the  following 
September,  in  whose  stable  conditions  a  pulse  of  121,200  on  the  21st, 
at  71°,  is  found.  It  disappears  October  5,  and  is  irregularly  present 
from  January  to  April,  with  larger  numbers  in  the  latter  part  of 
the  period.  It  is  not  found  in  1898  (Table  I.)  from  May  1  to  Decem- 

(14) 


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197 

ber  1,  but  is  continuously  present  in  the  winter  of  1898-99  from 
December  6  till  March  28,  when  collections  ceased. 

Male  eggs  were  recorded  but  once — April,  29,  1895 — and  there 
is  no  other  evidence  of  the  cycles  of  reproduction  beyond  the  pulses 
in  numbers.  They  suggest  a  polycyclic  habit  with  major  pulses 
in  spring  and  fall.  It  is  apparent  that  conditions  affect  these  cycles 
greatly,  as  is  seen,  for  example,  in  the  contrast  between  the  earlier 
years,  with  low  water  in  the  spring,  and  the  later  ones,  when  high 
water  was  longer  continued. 

This  variety,  rubens,  has  not  been  widely  reported  in  the  plank- 
ton. Skorikow  ( '96)  finds  it  in  June  in  the  River  Udy,  and  Kertesz 
( '94)  reports  it  from  Budapest,  while  Stenroos  ( '98)  finds  it  in  the 
littoral  fauna  of  Lake  Nurmijarvi  in  Finland,  and  also  in  the  plank- 
ton in  July  and  August. 

Brackionus  urceolaris  var.  bursarius  Barrois  and  v.  Daday.— 
Average  number  of  individuals,  206;  of  eggs,  33.  This  is  a  sum- 
mer variety,  and  forms  but  a  small  part — less  than  one  per  cent. — 
of  the  total  number  of  individuals  referred  to  the  species. 

Brachionus  variabilis  Hempel. — This  species  was  found  but  once 
in  1898,  but  was  more  abundant  in  former  years  (see  table  on  oppo- 
site page).  The  largest  development  which  it  attained  in  the  Illinois 
was  a  pulse  of  168,222  on  August  15,  1894,  at  84°.  The  largest 
number  in  subsequent  years  was  5,200  per  m.3  on  August  8,  1896. 
It  may  be  significant  of  the  connection  of  this  form  with  the  urceo- 
laris-rubens  group  that  the  great  pulse  of  1894  was  coincident  with 
an  unusual  development  of  rubens  on  that  date. 

This  species  is  a  summer  form,  the  earliest  record  being  May  24, 
1898,  at  74°,  and  the  latest  September  25,  1895,  at  73°.  Its  opti- 
mum temperatures  lie  near  the  summer  maximum.  If  this  form 
should  prove  to  be  merely  a  spinous  variety  of  B.  urceolaris  it  will 
afford  another  illustration  of  spinous  varieties  of  Brachionus  appear- 
ing at  high  temperatures,  in  accordance  with  the  hypothesis  of 
Wesenberg-Lund  ('00). 

In  Table  I.  there  is  given  for  1898  the  seasonal  distribution  of 
the  free  winter  eggs  of  Brachionus.  It  will  be  seen  that  they  occur 
throughout  practically  the  whole  year,  with  some  increase  after  the 
times  of  the  April-May  and  September  pulses. 

Cathy pna  leontina  Turner. — Average  number,  47,  in  1896,  a  year 
of  disturbed  hydrograph ;  less  abundant  in  previous  years,  and  not 


198 

recorded  in  subsequent  ones.  Earliest  record,  June  17,  at  76°;  and 
latest,  October  2,  at  63°.  Always  present  in  small  numbers  and 
evidently  adventitious. 

Cathy pna  luna  (Ehrbg.)  Gosse. — Average  number,  47.  Found 
in  every  month  but  November,  though  always  in  small  numbers  and 
irregularly.  All  but  six  of  the  thirty-three  records  fall  between 
April  1  and  October  3  and  above  50°.  Over  half  of  all  the  individu- 
als were  found  in  1896.  This  fact,  together  with  the  nature  of  the 
seasonal  distribution,  indicates  plainly  its  adventitious  character. 

Cathypna  rusticula  Gosse. — Found  once,  March  22,  1897,  at  44°. 
Not  previously  reported  from  American  waters. 

Ccelopus  porcellus  Gosse. — Average  number,  106.  From  March 
to  September,  at  37°  to  80°,  and  apparently  adventitious. 

Colurus  bicuspidatus  Ehrbg. — Average  number,  274.  This 
species  is  apparently  a  winter  planktont.  In  1897  it  appeared  first 
November  9,  at  50°,  and  was  found  somewhat  irregularly  through 
the  winter  until  May  17,  at  64°.  There  is  a  pulse  March  15,  at 
46°,  of  6,400.  Ovigerous  females  were  found  during  the  rise  of  the 
pulse,  and  males  on  April  12,  on  its  decline.  A  few  scattered  records 
were  made  in  the  following  winter,  beginning  November  8,  at  46°.  It 
occurs  in  the  plankton  during  flood  season  and  may  be  adventitious. 

Colurus  obtusus  Gosse. — Average  number,  38.  In  small  numbers 
and  irregularly  in  March  and  April  at  temperatures  below  50°,  and 
in  September  at  73°.  Hempel  ('99)  lists  also  C.  deflexus  Ehrbg. 

Diglena  circinator  Gosse. — Average  number,  121,  in  1896,  a  year 
when  many  adventitious  rotifers  were  brought  into  the  plankton 
by  disturbed  hydrographic  conditions.  All  the  records  lie  between 
April  29,  at  70°,  and  July  28,  at  81°.  An  ovigerous  female  was  found 
in  July.  The  species  is  adventitious  in  the  plankton. 

Diglena  forcipata  Ehrbg.  was  recorded  once — October  12,  1897, 
at  65°. 

Diglena  giraffa  Gosse  was  observed  but  once  in  the  river  plank- 
ton. ^Not  before  recorded  from  American  waters. 

Diglena  grandis  Ehrbg.  was  recorded  in  July  and  September  at 
76°  and  79°. 

Diglena  uncinata  Milne  was  found  August  12,  1898,  at  82°. 

Hempel  ('99)  reports  D.  biraphis  Gosse  and  D.  catellina  Ehrbg. 
in  waters  immediately  tributary  to  the  river.  All  members  of  the 


199 

genus  belong  to  the  littoral  fauna  among  vegetation,  and  are  adven- 
titious in  the  plankton  of  open  water. 

Euchlanis  pyriformis  Gosse. — Recorded  April  12,  1898,  at  52°. 
Hempel  ('99)  reports  it  from  June  to  October  in  collections  in  the 
river  in  1894  and  1895. 

Euchlanis  triquetra  Ehrbg. — Average  number,  19.  Found  irregu- 
larly from  July  to  November  at  84°  to  41°.  Hempel  ( '99)  reports  it 
also  in  June.  It  is  probably  adventitious. 

Hempel  ('99)  also  reports  E.  dilatata  Ehrbg.  in, the  river  from 
July  to  September,  and  E.  deflexa  Gosse  in  tributary  waters. 

Gastropus  stylifer  Imhof. — A  rotifer  doubtfully  referred  to  this 
species  was  found  sporadically  in  the  plankton  of  the  river.  It  was 
recorded  in  June,  1894,  and  July,  1896,  at  temperatures  above  75°. 
It  was  almost  continuously  present  in  1896  from  February  20  to 
April  10,  and  again  on  November  17  and  December  3.  It  did  not 
reappear  until  January  31,  1899,  from  which  time  it  continued 
present  until  the  close  of  operations  in  March.  Most  of  these  oc- 
currences are  at  minimum  temperatures  and  all  of  them  below  45°. 
I  have  followed  Weber  ('98)  and  Jennings  ('00)  in  using  Imhof 's 
name  Gastropus  stylifer  instead  of  Hudsonella  picta  Zach.  or  Notops 
pygmcEUs  Caiman,  by  which  names  the  species  has  been  frequently 
designated.  The  evidence  from  our  records  indicates  that  it  is  a 
somewhat  sporadic  winter  planktont  in  our  waters.  Lauterborn 
('93)  finds  it  to  be  a  perennial  planktont  in  the  Rhine,  with  its 
largest  numbers  in  summer. 

Hydatina  senta  Ehrbg.  was  found  September  20  at  73°.  Hempel 
('99)  also  reports  it  in  towings  from  the  river  in  March  and  July, 
1895.  This  species  is  very  common  in  European  waters,  but  has 
as  yet  been  found  in  America  only  in  the  Illinois  River  and,  by  Kel- 
licott  ('88),  at  Corunna,  Mich. 

Mastigocerca  bicornis  Ehrbg. — Average  number,  42.  Found 
irregularly  and  in  small  numbers  from  June  28  to  September  13 
above  63°.  Hempel  ('99)  reports  it  from  Quiver  Lake  among 
vegetation,  and  it  is  evidently  adventitious  in  the  river  plankton. 

Mastigocerca  bicristata  Gosse  was  found  but  once,  late  in  Septem- 
ber, 1895,  at  73°,  but  it  is  more  abundant  in  the  backwaters. 

Mastigocerca  carinata  Ehrbg. — Average  number,  1,674.  This 
species  was  present  in  the  plankton  from  the  middle  of  June  till  the 


200 

first  of  October,  and  at  irregular  intervals  and  in  small  numbers  in 
fall  and  winter  months.  The  distribution  in  years  prior  to  1898 
falls  within  the  limits  shown  in  Table  I.  In  this  year  the  bulk  of 
the  occurrences  lie  between  June  21  and  August  4,  and  above 
77°  and  72°.  The  optimum  lies  near  the  summer  maximum,  though 
occurrences  at  minimum  temperatures  in  March  and  December 
reveal  acclimatization  to  a  wide  range  of  temperatures.  In  this 
year  there  are  several  somewhat  irregular  pulses,  the  best-defined 
of  which  follow  the  pulses  of  chlorophyll-bearing  organisms  (cf. 
Table  I.  and  PI.  II.)  at  an  interval  of  one  or  two  weeks.  The  species 
was  not  recorded  so  frequently  in  previous  years,  in  some  of  which 
also  pulses  are  indicated.  These  pulses  are  not  consequent  upon 
floods,  and  the  species  is  apparently  not  adventitious  in  the  plankton 
but  a  normal  constituent.  Apstein  ('96)  reports  M.  capucina  as 
abundant  in  Dobersdorfer  Lake  from  June  to  October — a  seasonal 
distribution  similar  to  that  found  in  the  Illinois  River  for  M. 
carinata. 

Mastigocerca  elongata  Gosse  was  found  once — March  28,  1899,  at 
38°.  Hempel  ('99)  reports  it  in  June  in  Quiver  Lake. 

Mastigocerca  mucosa  Stokes  was  taken  in  August  to  October, 
1898,  at  82°- 62°,  in  small  numbers.  It  is  reported  by  Jennings 
('00)  as  "one  of  the  most  abundant  of  the  Rotifera  among  the 
vegetation  of  the  shallow  parts  of  Lake  Erie,"  but  it  was  not  reported 
by  Hempel  ('99)  in  similar  environment  about  Havana. 

Mastigocerca  stylata  Gosse  was  found -in  the  plankton  in  small 
numbers  in  June  and  July  at  temperatures  approaching  80°.  Hempel 
('99)  reports  it  also  in  August. 

In  addition  to  the  species  of  this  genus  above  listed,  Hempel  ('99) 
records  M.  lata  Jennings.  There  are  also  in  our  records  a  considera- 
ble number  of  individuals  referred  to  this  genus  but  not  specifically 
identified.  Many  of  these  belong  to  one,  or  possibly  several,  very 
small  species.  They  are  most  abundant  during  the  summer  months, 
reaching  a  pulse  of  16,800  on  June  28.  They  occur  in  large  numbers 
in  the  filter  collections  (average  for  1898,  798 ;  filter-paper,  145,384), 
and, it  seems, must  escape  with  ease  through  the  silk  net  on  account 
of  their  small  size  and  their  active  movements. 

A  number  of  species  in  this  genus  have  been  described  of  late 
from  the  fresh-water  plankton,  but  in  the  present  state  of  the  litera- 
ture of  the  subject  I  am  not  certain  to  what  species  these  forms 


201 

should  be  referred.  The  genus  is  sadly  in  need  of  critical  revision. 
It  includes  a  number  of  semi-limnetic  species,  whose  importance  in 
the  plankton  will  probably  be  revealed  by  more  perfect  methods  of 
collection. 

Metopidia  lepadella  Ehrbg.  was  found  only  in  March  and  June 
at  temperatures  above  46°.  It  is  apparently  adventitious. 

Metopidia  oblonga  Ehrbg.  was  found  once — July  29,  1895,  at  75°. 

Metopidia  salpina  Ehrbg.  wras  recorded  June  28,  1898,  at  78°. 

Metopidia  solidus  Gosse. — Average  number,  67.  This  is  the 
most  abundant  representative  of  the  genus  in  our  plankton.  It  was 
recorded  from  March  15  to  November  14,  at  temperatures  above 
45°.  Most  of  the  occurrences  are  in  the  summer  months  (Table  I.), 
at  maximum  temperatures.  The  numbers  are  small,  the  occurrences 
irregular,  and  the  species  evidently  adventitious. 

M.  rhomboides  Gosse  is  recorded  by  Hempel  ( '99)  from  the  river 
plankton,  as  also  M.  acuminata  Ehrbg.,  triptera  Ehrbg.,  and  bractea 
Ehrbg.  from  the  backwaters. 

Monostyla  bulla  Gosse. — Average  number,  50.  Present  in  small 
numbers  and  irregularly  from  April  till  the  middle  of  October  at 
temperatures  above  50°.  It  is  evidently  adventitious.  Jennings 
( '00)  finds. this  one  of  the  most  abundant  rotifers  among  the  aquatic 
vegetation  in  Lake  Erie.  It  is  in  our  waters  the  most  abundant  of 
the  genus  in  the  plankton,  especially  in  the  vegetation-rich  back- 
waters. 

Monostyla  lunaris  Ehrbg. — Average  number,  37.  Found  in  the 
extremes  of  the  temperature  range,  but  over  50  per  cent,  of  the 
occurrences  are  in  August-October.  Its  numbers  are  always  small 
and  its  occurrences  irregular.  It  is  plainly  adventitious. 

Monostyla  quadridentata  Ehrbg. — Average  number,  10.  This 
species  was  found  in  the  plankton  irregularly  in  July-September,  at 
maximum  temperatures.  It  is  abundant  (Hempel,  '99)  in  the 
backwaters,  where  vegetation  is  abundant,  and  is  apparently  adven- 
titious in  the  plankton.  In  addition  to  the  species  here  recorded 
Hempel  ('99)  lists  M.  cornuta  Ehrbg.  and  M.  mollis  Ehrbg.  from 
collections  in  the  river,  and  M.  dosterocerca  Schmarda  from  the  back- 
waters. This  is  an  exceedingly  variable  group,  and  will  repay  a 
thorough  revision  in  the  light  of  a  study  of  the  variation  of  its 
species.  A  considerable  reduction  in  the  number  of  these  so-called 
species  will  doubtless  result  from  such  a  study. 


202 

Noteus  quadricornis  Ehrbg. — Average  number,  19.  This  is  a 
rare  species  in  the  plankton,  being  found  in  1895  and  1896  in  July  at 
maximum  temperatures,  and  in  1898,  on  April  12,  at  52°,  and  on 
November  8,  at  46°. 

Notholca  longispina  Kell.— This  species,  which  has  been  found 
in  the  summer  plankton  of  many  European  and  American  waters, 
especially  our  Great  Lakes,  was  noted  but  once  in  the  Illinois — in 
January,  1895  (Hempel,  '99).  It  seems  to  prefer  cooler  and  purer 
waters. 

Notholca  striata  Ehrbg. — Average  number,  437,  including  varie- 
ties. This  is  a  winter  planktont  in  our  waters,  appearing  in  1897  on 
November  30,  at  34°,  reaching  a  maximum  of  10,840  March  22 
(Table  I.),  at  51°,  and  disappearing  April  19;  at  52°.  It  reappears 
the  following  autumn  on  November  1,  at  45°,  and  attains  a  maxi- 
mum of  4,000  March  21,  at  37°.  In  previous  years  the  occurrences 
all  lie  within  the  limits  of  November  1  and  April  24  with  the  excep- 
tion of  two  records  in  1895 — September  5  and  October  15,  at  74°  and 
56°.  The  spring  maximum  in  1896  (7,778)  was  on  April  10,  at 
52°,  and  in  1897  (4,260)  on  March  22,  at  43°.  In  each  year  but  a 
single  pulse,  that  of  March-April,  is  indicated.  Minor  fluctuations 
during  the  winter  (Table  I.)  are  in  some  cases  attributable  to  flood 
agencies. 

The  temperature  limits  of  this  species  are  quite  definitely  estab- 
lished. The  species  reappears  in  autumn  when  45°  is  reached,  and 
declines  rapidly  in  the  spring  after  50°  is  passed  and  is  but  rarely 
found  above  60°.  It  attains  its  greatest  numbers  late  in  winter  or 
early  in  spring  in  the  face  of  flood  conditions,  though  the  numbers 
attained  in  the  channel  waters  are  never  very  large. 

Empty  loricas  have  been  found  in  the  plankton  after  the  decline 
of  the  species  in  April,  and  females  with  a  single  egg  were  noted  in 
small  numbers  in  1895  during  the  rise  of  the  pulse. 

I  follow  the  suggestion  of  Weber  ('98)  that  N.  striata  should 
include  as  varieties  the  following:  N.  labis  Gosse,  N.  jugosa  Gosse, 
and  N.  acuminata  Gosse.  Examination  of  many  individuals  in  the 
plankton  proves  beyond  a  doubt  the  great  variability  of  the  organ- 
ism whose  seasonal  occurrence  we  have  traced.  It  varies  in  the 
length  of  the  posterior  spine,  in  the  proportions  of  the  lorica,  and 
in  the  development  of  the  striae  and  the  anterior  spines.  Of  a  total 
of  81 ,227  of  Notholca  striata  in  this  wider  sense,  68,887  were  referred 


203 

to  var.  acuminata,  3,852  to  var.  jugosa,  7,029  to  N.  striata  in  the 
narrower  sense,  and  1,469  to  other  varieties,  including  var.labis  and 
var.  scapha.  The  seasonal  distribution  of  N '.  striata  (sensu  strictu) 
and  var.  jugosa  lies  within  the  limits  of  that  of  var.  acuminata,  but 
occurrences  are  too  few  to  trace  their  seasonal  fluctuations. 

This  species  is  reported  by  Lauterborn  ('94)  in  the  winter 
plankton  of  the  Rhine.  He  also  notes  the  connecting  links  between 
N.  acuminata,  N.  striata,  and  N.  labis,  and  regards  them  as  belonging 
to  the  same  "  Formenkreis."  Apstein  ('96)  reports  N.  acuminata, 
N.  labis,  and  N.  striata  in  lakes  of  northern  Germany  and  indicates  a 
seasonal  distribution  which  coincides  closely  with  that  found  for 
these  forms  in  the  waters  of  the  Illinois.  He  also  reports  a  March- 
April  maximum  and  only  isolated  occurrences  in  midsummer. 
Forbes  ('83)  finds  the  species  in  the  stomachs  of  young  Coregonus 
feeding  upon  the  March  plankton  of  Lake  Michigan.  Seligo  ('00) 
also  finds  it  in  the  winter  plankton  of  Prussian  waters. 

Notommata  cyrtopus  Gosse  was  found  in  the  plankton  in  April 
and  September  at  temperatures  above  50°.  Hempel  ('99)  reports 
Ar.  aurita  Ehrbg.  from  the  river,  and  N.  tripus  Ehrbg.  and  N.  lacinu- 
lata  Ehrbg.  (  =  Diaschiza  lacinulata  Ehrbg.)  from  the  backwaters. 

Ploesoma  lenticulare  Herrick  was  found  in  the  plankton  of  the 
river  from  September  to  December,  1896,  throughout  the  whole 
range  of  temperatures  from  75°  to  the  winter  minimum.  Hempel 
( '99)  reports  it  from  May  to  December,  but  principally  in  vegetation 

Polyarthra  platyptera  Ehrbg. — Average  number  of  individuals, 
86,674;  of  eggs,  52,560.  In  1897,  94,653  and  58,235  ;  in  1896,  29,653 
and  11, 138;  in  1895,  28,947  and  20,074;  in  1894,  743  and  217.  The 
effect  of  the  stable  conditions  of  1897  and  of  the  recurrent  floods  of 
1896  is  seen  in  the  larger  averages  in  the  former  year  and  in  the 
smaller  ones  in  the  latter. 

This  is  one  of  the  most  abundant  rotifers  in  our  plankton,  includ- 
ing, as  it  does,  one  seventh  of  the  total  Rotifer  a,  and  exceeding  in 
numbers  all  other  species  of  the  group  excepting  only  Synch&ta 
stylata.  It  is  a  perennial  form,  and  was  recorded  in  every  plankton 
collection  but  two,  and  it  may  have  been  present  then. 

The  seasonal  distribution  of  this  abundant  species  is  very  char- 
acteristic of  the  form  which  most,  though  not  all,  plankton  organ- 
isms exhibit.  Two  prominent  features  are  (1)  a  limitation  of  large 
numbers  to  the  warmer  months  and  (2)  a  rhythmic  occurrence  of 


204 


recurrent  pulses  at  approximately  monthly  intervals.  In  Plate  V. 
I  have  plotted  the  seasonal  distribution  of  this  species  for  the  years 
1894-99.  The  plate  will  serve  as  one  of  the  best  illustrations  of 
the  nature  of  the  data  contained  in  my  statistical  records  that  could 
be  chosen  from  them.  It  illustrates  graphically  the  character  of 
the  seasonal  distribution  of  this  species  and  the  nature  of  what  I 
have  called  recurrent  pulses. 

In  the  table  which  follows,  as  elsewhere  in  similar  tables,  these 
pulses  are  listed  by  the  number  of  individuals  attained  at  their 
maxima,  and  are  located  according  to  the  dates  of  these  maxima. 

PULSES  OF  POLYARTHRA  PLATYPTERA. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1896 

Jan.   6 
"     25 

32° 
33° 

5,406 
2,736 

Feb.  25 

34° 

7.852 

Mar.  24 

41° 

57,267 

Tan      9? 

Feb   22 

39° 

1899 

Jan.  17 

33° 

20,800 

Feb.  14 

33° 

145,600 

Mar.     7 

33° 

71,200 

Year 

Date  * 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1896 

Apr.  24 

72° 

233,436 

May    8 

76° 

54,365 

June     1 
11 

69° 
73° 

18,000 
35,200 

1898 

Apr.  26 

57° 

696,000 

May  17 

64° 

195,200 

June  14 

82° 

432,800 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 

1896 

July  30 
July    6 

July  10 

"     28 

82° 
81° 

80° 
82° 

1,908 
231,504 

90  ,  000 
71,000 

Aug.     1 

"      21 

Aug.    8 

79° 
82° 

86° 

6,350 
117,513 

39,200 

Sept.  12 

79° 

19,272 

1897 
1898 

July  21 

81° 

172,000 

Aug.  24 

Aug.    2 
"      23 

78° 

78° 
82° 

230,400 

288,000 
96,000 

Sept.  14 
Sept.  27 

83° 
73° 

50,000 
238,400 

205 


PULSES  OF  POLYARTHRA  PLATYPTERA — continued. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 
1896 
1897 

Oct.  17 
Oct.  23 

58° 
51° 

1,140 
408 

Nov.  27 

33° 

74,942 

Dec.  18 
Dec.  29 
Dec.  14 

39° 
35° 
40° 

21,147 
37,560 
7,300 

Oct.     5 

71° 

816,000 

Nov.  15 

47° 

22,400 

1898 

Oct.  11 

"    25 

65° 

49° 

47,500 
37,500 

Nov.  22 

40° 

6,000 

Dec.  20 

33° 

63,400 

An  examination  of  this  table  and  the  graphic  presentation  (PI. 
V.)  of  the  seasonal  distribution  will  show  at  once  the  uniformly 
small  numbers  attained  at  low  temperatures.  Between  October 
15  and  April  15,  that  is  below  60°,  no  pulse  exceeding  100,000  is 
reached  save  one  of  122,400,  February  21,  1899,  at  33°.  Of  all  the 
records  in  this  period  only  seven  exceed  50,000.  On  the  other  hand, 
during  the  warmer  months,  above  60°,  the  pulses  have  a  much 
greater  amplitude.  Four  of  them  exceed  400,000,  and  there  are 
twenty-two  records  above  100,000.  The  summer  pulses  are  often 
separated  by  minima  which  approach  midwinter  levels,  but  in  spite 
of  this  the  general  level  of  summer  occurrences  is  much  higher  than 
that  of  the  colder  season.  In  1898  the  average  from  April  15  to 
October  15  was  30,861  per  m.3,  and  for  the  other  months  of  the  year, 
15,813,  or  about  half  the  number  in  the  warmer  season.  From 
these  facts  of  distribution  it  is  apparent  that  though  perennial  the 
species  finds  its  optimum  conditions  at  temperatures  above  60°.  The 
statement  of  Hempel  ('99)  that  it  thrives  best  in  cold  water  is  not 
borne  out  by  the  statistical  examination  in  any  of  the  years. 

The  recurrent  pulses  of  this  species  vary  greatly  in  amplitude. 
The  largest  pulse  recorded  was  that  of  816,000,  October  5,  1897,  at 
71°.  It  appeared  in  a  period  of  prolonged  low  water  and  at  the 
close  of  one  of  high  temperatures  continued  beyond  the  usual 
September  limit  (Pt.  I.,  PL  XL),  in  a  very  unusual  development  of 
Carteria  and  the  smaller  algae  of  the  water-bloom  (PL  II.).  Similar 
autumnal  pulses  do  not  appear  in  other  years,  the  autumnal  develop- 
ment as  a  rule  not  exceeding  to  any  noticeable  degree  that  of  mid- 
summer. There  has  been  in  every  fully  tested  spring  a  large  vernal 
pulse,  usually  at  the  time  of  the  spring  volumetric  maximum,  or 
thereabouts.  In  1896  and  1898  it  was  the  largest  pulse  of  the  year. 


206 

This  was  not  true  in  other  years,  but  collections  in  those  years  were 
too  infrequent  to  trace  the  seasonal  distribution  of  the  species  with 
accuracy  at  that  season.  It  is  volumetrically  of  some  importance 
in  determining  the  quantitative  fluctuations  in  the  total  plankton. 
Computations  based  on  its  average  size  indicate  that  approximately 
600,000,  including  eggs,  would  be  required  to  form  1  cm.3  of  plank- 
ton. On  this  basis,  and  allowing  10  per  cent,  for  interstices,  it 
constituted  at  the  time  of  its  vernal  maximum  in  1898  about  10 
per  cent,  of  the  total  volume  of  the  plankton  (silk-net  catch). 

The  table  on  pages  204  and  205  lists  43  pulses,  of  which  6  lie  out- 
side of  the  period  included  in  Plates  I.  and  II.  Of  the  38  remaining 
pulses'  16  coincide  in  location  with  the  whole  or  a  part  (in  case  of 
divided  culminations)  of  the  pulses  of  the  chlorophyll-bearing  organ- 
isms; 12  follow  at  the  next  collection,  usually  at  intervals  of  one 
week ;  and  6,  after  a  fortnight.  The  remaining  4  do  not  bear  this  rela- 
tion, occurring  in  autumn  or  midwinter,  when  all  pulses  were  feeble 
and  ill-defined.  A  comparison  of  Plates  I.  and  II.  with  V.  will 
show  that  not  all  of  the  chlorophyll-bearing  pulses  are  attended  by 
pulses  of  Polyarthra;  nor  is  there  any  constant  relation,  excepting 
the  vernal  pulse,  between  the  size  of  the  pulses  of  the  two  groups 
of  planktonts  in  question.  Nevertheless,  the  dependence  of  the 
recurrent  periods  of  rapid  multiplication  of  Polyarthra  upon  the 
rhythmic  occurrences  of  the  chlorophyll-bearing  organisms  upon 
which  they  largely  depend  for  their  food  is  strongly  suggested  by 
the  data  here  offered.  Food  relations  thus  dominate  the  repro- 
ductive cycles. 

The  pulses  of  Polyarthra  form  a  considerable  portion  of  many  of 
the  pulses  of  the  total  Ploima,  and  it  is  but  natural  that  we  should 
find  a  coincidence  in  their  locations.  This  may  be  followed  for  1898 
in  Table  I .  In  a  number  of  instances  the  culminations  of  the  pulses 
are  not  exactly  coincident,  but  separated  by  the  interval  between 
two  collections.  The  association  of  the  two  pulses  is,  however, 
apparent  in  every  case,  and  a  similar  relation  may  be  traced  in  prior 
years. 

These  recurrent  pulses  afford  evidence  for  the  polycyclic  habit 
of  this  species.  Additional  proof  of  this  phenomenon  is  found  in  the 
evidences  of  sexual  reproduction — either  male  or  winter  eggs 
attached  to  the  female — which  have  attended  many  of  the  pulses. 
The  eggs  of  this  species,  both  summer  and  winter  forms,  are  very 


207 

readily  detached  in  the  manipulation  of  the  plankton,  so  much  so 
that  in  1898  less  than  6  per  cent,  remained  attached.  More  or  less 
uncertainty  attends  the  determination  of  the  parentage  of  detached 
winter  and  male  eggs,  so  that  decisive  proof  of  sexual  reproduction 
is  best  obtained  from  the  attached  eggs.  In  Table  I.  will  be  found 
the  records  of  free  and  attached  male  and  winter  eggs  recorded  in 
1898.  Evidence  will  be  found  in  this  of  sexual  reproduction  at- 
tending the  pulses  of  March,  April,  May,  September,  and  December. 
The  presence  of  winter  eggs  at  intervals  throughoutrthe  greater  part 
of  the  year  may  be  due  either  to  their  continual  production  or,  as 
seems  more  probable,  to  their  continuance  in  the  plankton  for  some 
time  after  their  formation.  The  presence  of  attached  winter  eggs, 
or  of  larger  numbers  of  free  winter  eggs,  seems  to  mark  the  culmina- 
tion and  decline  of  the  pulse.  Male  eggs,  on  the  other  hand,  are 
more  generally  present  during  both  the  rise  and  decline  of  the  pulses. 
Somewhat  similar  evidence  of  sexual  cycles  attends  many  of  the 
larger  pulses  in  years  prior  to  1898. 

This  species  affords  a  striking  example  of  a  perennial  eulimnetic 
planktont.  It  is  found  in  midwinter  under  the  ice  in  water  at  the 
freezing  point,  and  even  under  these  conditions  it  multiplies,  pro- 
ducing pulses  whose  amplitude  surpasses  that  of  many  rotifers  of  the 
plankton,  and  runs  a  reproductive  cycle  similar  to,  though  of  less 
amplitude  than,  those  at  other  seasons  of  the  year.  It  shares  with 
other  organisms  the  vernal  outburst,  and  repeats  the  process  in 
summer  months  under  maximum  conditions  of  heat  and  in  waters 
whose  chemical  condition  is  very  different  from  that  in  which  the 
hiemal  and  vernal  pulses  appeared.  Successive  generations  of  this 
species  are  thus  adapted  to  widely  different  conditions.  Through 
all  the  changes  incident  to  ice,  stagnation,  flood,  sewage  pollution, 
changing  temperature,  the  wax  and  wane  and  change  of  food,  the 
constant  and  unceasing  warfare  of  enemies  which  prey  upon  it  and 
of  parasites  which  plague  it,  and,  above  all  and  continuously,  the 
removal  of  countless  individuals  from  the  place  of  their  origin  by 
the  ceaseless  current  of  the  stream,  this  species  lives  on,  holds  its 
own  in  the  plankton,  and  repeats  year  after  year  the  same  sequence 
of  rhythmic  pulses  of  occurrence  in  the  river  water.  The  secret  of 
the  process  doubtless  lies  in  its  capacity  to  produce  repeatedly  these 
crops  of  winter  eggs  which  serve  to  seed  the  environment  and  start 


208 

anew  the  cycle  of  growth  and  reproduction  whenever  the  favorable 
conditions  prevail. 

There  is  in  this  species  no  hard  lorica  whose  variable  processes 
might  serve  to  demonstrate  to  every  observer  its  capacity  for  varia- 
tion. This  is  doubtless  one  of  the  reasons  why  we  do  not  find  a  host 
of  new  species  and  varieties  oiPolyarthra  as  in  the  case  of  Brachionus. 
It  is  subject  to  considerable  variation  in  size,  and  the  swimming 
lamellae  vary  in  length,  width,  and  serrations.  Hempel  ( '99)  records 
Wierzejski's  var.  euryptera  in  our  plankton,  and  I  have  often 
observed  it,  but  no  record  was  kept  of  it  since  the  characters  which 
define  it  are  not  readily  seen  in  plankton  enumeration.  Weber  ( '98) 
has  mentioned,  without  designating  by  name,  a  long-spined  variety 
which  I  find  very  common  among  the  individuals  which  occur  in 
the  Illinois. 

This  planktont  is  subject  to  attacks  of  internal  parasites  (Sporo- 
zoaf)  which  infest  it  at  the  times  of  its  maximum  pulses,  though 
never  to  the  extent  observed  in  the  case  of  Bimcerium  in  Brachionus. 
It  is  very  frequently  loaded  down  by  Colacium,  and  some  of  the 
smaller  peritrichous  Ciliata  are  often  found  upon  it.  The  absence 
of  a  hard  lorica  has  served  to  obscure  somewhat  its  food  relations 
to  whatever  animals  prey  upon  it. 

Polyarthra  platyptera  is  a  cosmopolite,  and  is  apparently  found 
generally  in  the  fresh-water  plankton.  Jennings  ( '00)  reports  it  as 
abundant  in  the  waters  of  the  Great  Lakes,  and  it  has  been  found 
generally  in  American  waters.  Zacharias  ('98)  and  Marsson  ('00) 
find  it  in  pond  and  stream  waters  of  Germany;  Stenroos  ('98) 
reports  it  as  a  predominant  rotifer  in  the  plankton  and  littoral  regions 
of  Finland  waters ;  and  Borge  ( '00)  finds  it  in  Swedish  plankton.  It 
has  also  been  found  to  be  an  important  constituent  in  the  plankton 
of  European  streams.  Skorikow  ('96)  finds  that  it  is  the  most 
abundant  rotifer  in  the  summer  plankton  of  the  River  Udy,  consti- 
tuting almost  a  third  of  the  total  rotifers.  There  are  indications 
in  his  records  of  recurrent  pulses,  and  the  largest  numbers  are  found 
in  September.  Zimmer  ('99)  finds  it  perennial  in  the  Oder,  but 
never  abundant.  Schorler  ( '00)  finds  it  in  the  Elbe  from  April  to 
September,  with  maximum  in  August.  Lauterborn  ('98a)  lists  this 
species  among  the  perennial  rotifers,  and  states  that  it  is  dicyclic  in 
the  Rhine  and  its  adjacent  waters,  which  he  has  examined  quite 
thoroughly.  The  vernal  sexual  period  begins  with  the  appearance 


209 

of  the  male  eggs  in  March,  and  winter  eggs  follow  in  April  and  May. 
The  second  sexual  period  extends  from  the  end  of  July  to  the  end 
of  October,  with  a  maximum  in  September-October.  This  bears 
some  resemblance  to  the  distribution  in  the  Illinois,  with  the 
exception  that  the  recurrent  cycles  which  make  the  species  poly- 
cyclic  were  not  noted,  and  that  male  or  winter  eggs  were  not  present 
in  the  colder  months.  It  may  be  that  the  application  of  the 
quantitative  statistical  method  with  brief  intervals  of  collection  in 
the  Rhine  would  reveal  a  still  closer  correspondence  in  the  seasonal 
routine  of  Polyarthra  in  the  two  streams.  Wesenburg-Lund  ('98) 
finds  that  temperature  has  nothing  to  do  with  the  appearance  of 
the  sexual  cycle  of  this  species  in  Danish  waters.  Males  were 
found  in  December,  as  also  (eggs  only)  in  the  Illinois.  He  also 
found  differences  in  different  bodies  of  water  as  to  the  times  of  the 
sexual  cycles.  Apstein  ('96)  has  found  this  species  perennial  and 
one  of  the  most  abundant  rotifers  in  plankton  of  the  lakes  near 
Plon,  Germany,  with  maximum  period  from  April  to  August,  and 
in  November  in  one  lake,  and  in  July-August  in  another.  The 
sexual  cycle  was  noted  in  May- June  only.  Seligo  ('00)  finds  the 
species  perennial  in  lakes  near  Danzig,  with  large  numbers  in  April 
and  July.  His  collections  were  too  widely  separated  to  trace  fully 
the  seasonal  fluctuations.  Burckhardt  ('OOa)  finds  Polyarthra  in 
small  numbers  in  winter  months  in  the  plankton  of  Swiss  lakes,  and 
in  larger  numbers  in  the  summer,  but  does  not  trace  their  seasonal 
fluctuations. 

Pterodina  patina  Ehrbg. — Average  number  of  females,  37.  With 
two  exceptions  all  the  records  of  this  species  lie  between  the  last  of 
May  and  the  first  of  October.  There  are  but  four  records  below 
70°.  This  indicates  optimum  conditions  for  the  species  during  the 
period  of  maximum  heat,  and  further  evidence  of  this  lies  in  the 
occurrence  of  the  larger  numbers  during  this  period.  Appearances 
in  January-March  suggest  a  perennial  habit ;  and  small  and  irregular 
numbers,  that  the  species  is  largely  adventitions.  Hempel  ('99) 
also  records  P.  valvata  Hudson  from  Quiver  Lake. 

Rattulus  tigris  O.  F.  Mull. — Average  number  of  females,  207.  I 
have  not  found  this  species  in  any  year  later  than  October,  though, 
as  shown  in  Table  I. ,  it  appears  in  January  at  minimum  temperatures, 
and  continues  in  small  numbers  and  somewhat  irregularly  until 
autumn.  These  conditions  and  the  absence  of  pulses  suggest  that 


210 

the  species  is  adventitious  in  the  plankton.  The  greater  part  of  the 
occurrences  were  recorded  above  50°  and  the  larger  numbers  above 
60°,  indicating  an  optimum  during  summer  months.  The  record  in 
Table  I.  refers  to  the  species  figured  by  Jennings  ('00)  under  this 
name. 

Rattulus  sulcatus  Jennings  was  found  seven  times  in  the  plankton 
in  July  and  August  during  maximum  temperatures.  It  is  probably 
adventitious  in  the  plankton. 

Salpina  brevispina  Ehrbg.  was  found  September  5,  1895,  at 
74°,  and  April  29,  1896,  at  70°. 

Salpina  eustala  Gosse  was  found  July  13,  1894,  at  82°. 

Salpina  macracantha  Gosse  was  found  September  5,  1895,  at  74°. 

Salpina  ventralis  Ehrbg.  was  found  July  29,  1895,  at  75°.  In 
common  with  other  species  of  the  genus  it  is  adventitious  in  the 
plankton. 

Schizocerca  diversicornis  v.  Daday. — Average  number  of  females, 
46.  The  earliest  record  of  this  species  was  June  1,  1896,  at  70° ;  and 
the  latest,  September  20,  1895,  at  78°.  Most  of  the  records  and 
the  larger  numbers  are  in  July-September  during  the  period  of 
maximum  heat,  in  which  its  optimum  conditions  must  be  found. 
Egg-bearing  females  were  also  found  in  these  months.  This  species 
is  closely  related  to  the  Anuroza  aculeata  group,  and  like  it  is  exceed- 
ingly variable,  especially  in  degree  of  development  of  the  various 
spines.  Variety  homoceros  Wierz.  was  found  in  May,  June,  and 
August,  1896.  Five  sixths  of  all  the  individuals  recorded  were 
found  in  1896,  and  the  fact  that  this  was  a  year  of  unusually  dis- 
turbed hydrograph  (Pt.  I.,  PI.  X.)  suggests  that  this  form  may  be 
to  some  extent  adventitious  in  our  plankton,  but  no  direct  relation 
to  the  access  of  flood  waters  can  be  traced. 

Lauterborn  ('98a)  lists  this  species  among  the  summer  planktonts 
of  the  Rhine,  and  Seligo  ( '00)  finds  it  in  large  numbers,  with  a  maxi- 
mum in  July,  in  lakes  near  Danzig.  Zacharias  ('98)  reports  it  in 
German  pond  plankton,  Zimmer  ('99)  finds  it  in  the  Oder,  and 
Schorler  ( '00)  in  the  summer  plankton  of  the  Elbe. 

Synchazta  pectinata  Ehrbg. — Average  number  of  individuals, 
3,950;  of  eggs,  13,823.  It  was  much  more  abundant  in  previous 
years,  averaging  in  1897  23,227  and  28,230;  in  1896,  7,064  and 
7,927;  in  1895,  13,071  and  4,730;  in  1894,  7,520  and  1,659.  The 
effect  of  the  disturbed  hydrograph  of  1896  is  seen  in  the  smaller 


211 


numbers  of  that  year,  while  the  larger  numbers  in  1897  may  be 
attributed  to  the  more  stable  conditions.  The  small  numbers  in 
1898  do  not  seem  to  be  correlated  with  any  feature  of  the  environ- 
ment. 

This  species  has  been  found  in  every  month  of  the  year,  and  is 
thus  perennial  in  our  plankton.  As  will  be  seen,  however,  in  Table  I. , 
the  most  of  the  occurrences  and  a  much  greater  proportion  of  the 
individuals  are  found  between  May  and  October,  and  thus  above 
60°.  The  same  limitations  are  found  in  the  other  years,  with  the 
exception  that  in  1896  there  was  a  more  continuous  and  larger  de- 
velopment from  the  last  of  February.  In  the  table  which  follows 
it  may  be  noted  that  all  of  the  pulses  but  four  are  at  temperatures 
above  70°,  and  of  these  four  none  exceeds  25,000,  and  two  do  not 
exceed  2,500.  The  optimum  conditions  for  the  species  in  our 
waters  are  therefore  above  70°.  The  average  temperature  at  the 
time  of  the  larger  pulses  is  near  80°.  The  vernal  pulses  are  poorly 
defined,  as  are  likewise  the  autumnal  ones.  It  is  a  midsummer 
species  in  our  waters,  with  its  maximum  in  August. 

PULSES  OF  SYNCH^ETA  PECTINATA. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1895 

1896 

Mar  3 

35° 

6   360 

1897 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 

Tiilv  1  3 

83° 

74  606 

1895 
1896 

July  23 
Tnlv  in 

80° 
80° 

1,749 
22   200 

Aug.  12 

85° 

7ro 

175,230 

Sept.  12 

79° 

27,740 

"     28 

82° 

38,000 

1898 
1898 

July  19 

84° 
350 

20,800 

"      24 

Aug.    2 
"      23 

78° 

78° 
82° 

264,000 

12,000 
3,200 

Sept.  27 

73° 

30,400 

(IS) 


212 

Of  the  18  pulses  listed  in  the  preceding  table  17, fall  within  the 
limits  of  periods  included  in  Plates  I.  and  II.  Of  these  17  there  are 
7  which  coincide  with,  and  9  which  follow  shortly  after,  -the  culmina- 
tion of  the  pulses  of  the  chlorophyll-bearing  organisms,  while  1,  a 
small  one  in  March,  1896,  shows  no  such  correlation.  Food  is  thus 
a  primary  factor  in  the  production  of  these  recurrent  pulses.  -As 
will  be  seen  in  Table  I., these  pulses  uniformly  coincide  with  those 
of  the  total  Ploima,  and  a  similar  relation  may  be  followed  in  pri<  >r 
years. 

The  eggs  of  this  species  are  not  usually  carried  by  the  female  for 
any  length  of  time,  and  are  rarely  found  attached  in  preserved 
material.  For  this  reason  the  sexual  cycles  are  not  easily  followed 
with  accuracy  in  the  statistical  data.  It  may  be  seen  in  Table  I.  that 
the  free  winter  eggs  belonging  to  both  species  of  SynchcBta  are  most 
numerous  in  the  period  of  the  larger  pulses,  and  that  their  occur- 
rences show  some  tendency  to  coincide  with  these  pulses.  Proof 
that  these  pulses  terminate  in  sexual  reproduction  is  thus  lacking, 
though  it  seems  probable  from  some  of  the  evidence. 

Synch&ta  pectinata  has  not  been  widely  reported  from  American 
waters.  Jennings  ('94)  finds  it  in  Michigan  and  Kellicott  ('97)  in 
Lake  Erie,  but  it  has  not  been  elsewhere  reported  in  American 
plankton.  It  appears,  however,  in  many  European  records.  •  Skori- 
kow  ('96)  finds  it  in  the  summer  plankton  of  the  River  Udy,  in 
Russia ;  Zimmer  ( '99)  finds  it  in  common  with  5.  tremula  in  the  Oder 
throughout  the  year.  He  makes  the  statements  that  it  is  never 
rare,  is  somewhat  more  abundant  in  the  spring,  and  is,  at  other 
times,  present  "in  relativ  gleichmassiger  Haufigkeit."  In  the 
light  of  our  results  it  seems  probable  that  the  data  at  Zimmer 's 
disposal  were  insufficient  to  justify  his  conclusions  as  to  the  uniform- 
ity of  its  seasonal  distribution.  Schorler  ('00)  finds  it  in  the  Elbe 
in  April,  May,  and  October,  with  a  maximum  in  May.  Lauterborn 
( '98a)  finds  it  perennial  in  the  plankton  of  the  Rhine,  and  lists  it 
among  the  dicyclic  species  with  two  periods  of  sexual  reproduction, 
one  in  April  and  one  from  the  end  of  July  to  October.  Judging  from 
the  character  of  the  statistical  data  which  have  been  presented  for 
this  and  other  species  in  the  Illinois  it  seems  probable  that  the  later 
period  noted  by  Lauterborn  may  include  several  cycles,  and  that 
the  species  is  usually  a  polycyclic  one.  Seligo  ('00)  reports  it 
perennial  in  waters  near  Danzig,  with  largest  numbers  in  April  and 


213 


September.  Apstein  ('96)  finds  that  this  species  (including  5. 
tremula  and  5.  grandis]  is  one  of  the  most  abundant  in  lakes  near 
Plon,  with  variable  maxima  in  different  bodies  of  water.  He  finds 
it  perennial  in  one  case,  and  reports  vernal  maxima.  Winter  eggs 
were  found  in  March  and  April. 

Synch&ta  stylata  Wierz. — Average  number  of  individuals,  120,391 ; 
of  eggs,  17,797.  In  1897,  42,577  and  9,127;  in  1896,  24,099  and 
5,125;  in  1895,  155,880  and  2,418;  in  1894,  8,582  and  132.  This 
species  affords  an  exception  to  the  general  rule  hitherto  observed 
among  the  rotifers  of  our  plankton  in  that  it  is  more  abundant  in 
1898  than  in  the  previous  year.  As  will  be  seen  in  the  following 
table  both  the  vernal  and  autumnal  pulses  are  unusually  large  in 
1898,  while  in  the  previous  year  the  vernal  pulse  is  only  moderate 
and  the  autumnal  pulse  is  scarcely  to  be  detected.  For  some 
reason  the  prolonged  low  water  and  sewage  contamination  of  the 
autumn  of  1897  was  not  favorable  to  the  usual  growrth  of  this 
species.  It  may  be  that  it  was  crowded  out  by  the  unusual  develop- 
ment of  Polyarthra  at  that  season  (PL  V.). 

PULSES  OF  SYNCH^ETA  STYLATA. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

"       25 

33° 

3,648 

330 

1899 

Jan.    14 

34° 

12,000 

Feb.   14 

32° 

19,200 

"      22 
Mar.  21 

51° 
37° 

58,000 
5,600 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1896 

Apr.  29 

70° 

380,586 

May  25 

75° 

10,800 

June  17 

76° 

79,200 

60° 

1    139  000 

June  21 

77° 

795   200 

"      31 

70° 

61,600 

214 


PULSES  OF  SYNCH/ETA  STYLATA — continued. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1894 
1895 
1896 
1897 
1898 

Aug.     1 
Aug.     8 

79° 
86° 

10,287 
8,400 

Sept.  27 

73° 

12,225 

July   21 
July   19 

81° 
84° 

103,200 
64,800 

Sept.    7 
Sept.  27 

80° 
73° 

28,000 
265,600 

Aug.     2 
"      23 

79° 
82° 

170,400 
24,800 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1  894 
1895 
1896 
1897 

Oct.    17 

58° 

63,935 

Nov.  27 
Nov.  17 

Nov.    9 
"      30 

33° 
44° 

50° 
34.5° 

901,901 

114,000 

26,400 
87,200 

Dec.   11 

32° 

1,121,056 

Oct.      5 
"      19 

71° 
65° 

12,000 
15,800 

Dec.  14 

36° 

72,200 

1898 

Oct.   25 

49° 

824,500 

Nov.  15 

41° 

110,000 

Dec.     6 

20 

34° 
33° 

42,500 
59,200 

This  is  the  most  abundant  of  all  the  rotifers  in  our  plankton, 
exceeding  by  30  per  cent.  Polyarthra,  the  next  in  abundance.  It 
constituted  one  fifth  of  the  total  Ploima  in  1898,  and  is  accordingly 
a  large  factor  quantitatively  and  ecologically  in  the  economy  of  the 
plankton  of  the  Illinois  River. 

It  is  a  perennial  planktont,  occurring  in  six  sevenths  of  our 
collections  and  usually  in  considerable  numbers.  The  distribution 
in  1898  (Table  I.)  is  a  fair  index  of  the  usual  seasonal  routine,  with 
the  exception  that  in  all  prior  years  the  July-August  minimum  is 
more  pronounced  and  better  sustained.  The  development  in 
January-February  is  never  large,  rarely  exceeding  20,000.  In 
March,  numbers  rise  rapidly,  usually  with  a  minor  pulse,  the  re- 
covery from  which  in  April  culminates  in  a  vernal  pulse,  which  in 
three  of  the  six  years  was  the  largest  of  the  year.  Following  this 
vernal  pulse  there  is  a  series  of  smaller  pulses  throughout  the  sum- 
mer. The  decline  of  the  June  flood,  when  this  occurs,  seems  to  offer 
favorable  conditions  (cf.  foregoing  table  and  Pt.  I.,  PI.  IX.-XII.) 
for  the  development  of  a  pulse  which  is  but  little  smaller  than  the 
vernal  one.  It  may  be  of  some  significance  that  this  pulse  and  the 


215 

vernal  one  both  occur  on  the  decline  of  the  major  floods  of  the 
year,  and  that  the  relative  proportions  of  the  two  floods  are  to  some 
degree  paralleled  by  the  amplitude  of  the  pulses  of  Synch&'ta  which 
attend  their  decline.  The  effect  of  the  impounding  backwaters  as 
reservoirs  for  the  greater  development  of  the  plankton  is  suggested 
by  these  data. 

Following  the  midsummer  minimum  is  an  autumnal  pulse  whose 
amplitude  and  location  alike  are  subj  ect  to  much  variation.  As  will  be 
seen  in  the  table  on  pages  213  and  214,  the  maximum jmtumnal  pulse 
is  located  twice  in  October,  twice  in  November,  and  once  in  Decem- 
ber. This  may  be  due  to  the  fact  that  the  collections  are  insufficient 
in  some  of  the  years,  or  to  the  probability  that  any  one  of  several 
recurrent  autumnal  pulses  may  be  the  major  pulse  of  that  season. 

An  examination  of  the  seasonal  distribution. in  1898  (Table  I.) 
and  of  the  location  and  temperatures  of  the  pulses  recorded  in  the 
table  on  pages  213  and  214  will  suffice  to  demonstrate  the  capacity  of 
this  species  to  develop  at  all  temperatures  within  the  seasonal  range. 
The  largest  pulse  (1,139,000  on  May  3,  1898)  is  at  60°,  and  the  next 
in  size  (1,121,056  on  December  11,  1895)  is  at  32°.  It  will,  however, 
be  seen  in  the  two  tables  that  the  pulses  and  the  numbers  in  general 
during  the  periods  of  maximum  heat  and  cold  are  not  so  large  as  in 
the  intervals  of  more  moderate  temperatures.  The  impetus  of  the 
autumnal  development  may  carry  some  of  the  pulses  over  in  to 
minimum  temperatures,  but  the  level  of  development  declines 
thereafter.  There  is  thus  something  of  a  tendency  for  the  average 
temperature  of  the  larger  occurrences  to  approach  the  average 
temperature  of  the  year. 

The  number  of  pulses  listed  in  the  table  on  pages  213and214is38. 
Of  these,  34  fall  within  the  period  included  in  Plates  I.  and  1 1.  of  the 
pulses  of  chlorophyll-bearing  organisms.  Of  the  34  there  are  18 
which  coincide  in  location  with  these  plant  pulses,  12  which  follow 
at  a  brief  interval,  and  4  which  bear  no  such  relation,  three  of  the 
last  being  minor  winter  pulses. 

The  dependence  of  the  recurrent  periods  of  rapid  multiplication 
of  Synchata — the  most  abundant  rotifer  of  the  plankton — upon  the 
rhythmic  increase  of  the  food  supply  is  thus  fairly  demonstrated. 
The  coincidence  of  the  pulses  of  Synch&ta  with  those  of  the  total 
Ploima  is  readily  seen  in  Table  I., and  is  equally  apparent  in  prior 
vears. 


216 

Eggs  of  this  species  are  not  carried  by  the  parent  for  any  length 
of  time,  so  that  reproductive  cycles  are  not  easily  traced.  The  total 
number  of  the  summer  eggs  of  Synchceta  will  be  found  (Table  I.)  to 
fluctuate  somewhat  with  the  pulses  of  the  species.  The  free  winter 
eggs,  belonging  probably  to  both  species  of  Synchatta,  also  show 
some  tendency  to  predominate  at  and  after  the  culmination  (Table 
I.)  of  the  pulses.  A  female  carrying  a  male  egg  wTas  recorded  during 
the  rise  of  the  spring  pulse  in  1898,  and  attached  winter  eggs  were 
noted  at  the  vernal  pulse  in  1895  and  1897.  The  evidence  points 
toward  the  culmination  of  these  pulses  in  a  sexual  cycle. 

The  soft  and  flexible  nature  of  this  rotifer  and  the  absence  of 
spinous  outgrowths  have  made  whatever  variability  the  species 
possesses  less  evident  than  it  is  in  such  a  genus  as  Brachionus.  There 
is  considerable  variation  in  size — possibly  due  to  age — even  in  the 
same  collection.  The  determination  of  preserved  material  of  this 
genus  is  fraught  with  insuperable  difficulty.  The  separation  of 
pectinata  and  stylata  in  our  records  is  at  the  best  only  probable.  It 
may  be  that  other  species  of  Synchceta  have  been  included  writh  the 
individuals  referred  to  stylata.  In  any  event  the  result  of  the 
division  has  led  to  symmetrical  results  comparable  with  those  of 
other  planktonts.  Synch&ta  is  often  parasitized  at  the  times  of  the 
larger  pulses  by  some  sporozoan  (?).  At  the  maximum  of  the 
vernal  pulse  in  1898  over  4  per  cent,  of  the  individuals  were  thus 
affected,  the  infestation  continuing  through  the  decline  of  the  pulse. 
External  parasites,  Colacium  and  Rhabdostyla,  are  rare. 

This  species  has  not  been  found  widely  in  the  plankton,  possibly 
because  of  the  confusion  of  stylata,  tremula,  and  pectinata  in  identifi- 
cation. From  the  large  numbers  reported  in  almost  every  instance 
where  it  has  been  found,  the  expectation  of  its  wide-spread  occur- 
rence is  at  least  raised,  waiving  in  this  connection  the  possibility  of 
specific  confusion.  Jennings  ( '94)  found  it  to  be  very  abundant  in 
towings  in  Lake  St.  Clair,  and  ( '96)  in  Lake  Michigan  near  Charle- 
voix.  He  finds  it  less  abundant  in  the  summer  plankton  of  Lake 
Erie  ('00).  Stenroos  ('98)  reports  it  as  one  of  the  most  abundant 
limnetic  rotifers  in  Lake  Nurmijarvi  in  Finland  in  the  summer,  and 
Skorikow  ( '97)  finds  that  next  to  Polyarthra  it  is  the  most  abundant 
rotifer  in  summer  months  in  the  River  Udy  near  Charkow,  Russia. 
His  figures  of  occurrence  show  some  traces  of  recurrent  cycles  in 
these  months,  with  maximum  numbers  at  the  first  of  August.  Lau- 


217 

terborn  ('98a)  lists  it  among  the  summer  rotifers  of  the  plankton 
of  the  Rhine.  The  genus  is  in  need  of  a  thorough  revision  in  the 
light  of  possible  variation.* 

Taphrocampa  annulosa  Gosse. — Average  number,  71.  Found 
in  September,  at  73°.  Evidently  adventitious. 

Triarthra  longiseta  Ehrbg. — Average  number  of  individuals, 
3,147;  of  eggs,  293.  This  species  was  about  twice  as  abundant  in 
the  stable  conditions  of  1897,  and  was  present  in  less  than  half  these 
numbers  in  the  recurrent  floods  of  1896. 

It  is  a  perennial  species,  having  occurred  in  every  month  of  the 
year.  The  continuous  occurrences  and  the  larger  numbers  lie  in 
all  years  between  May  and  October  and  above  60°.  In  1898,  only 
about  3  per  cent,  of  the  total  individuals  were  found  below  this 
temperature.  With  the  exception  of  the  vernal  pulse  of  1898  all 
of  the  larger  numbers  were  found  in  the  period  of  maximum  heat. 
The  optimum  conditions  for  this  species  are  thus  found  within  that 
period  and  above  70°. 

The  seasonal  routine  of  the  species  is  varied  somewhat  from  year 
to  year.  There  is  usually  a  slight  vernal  pulse — larger  than  usual 
in  1898 — and  this  is  followed  by  recurrent  pulses  throughout  the 
summer.  The  season  closes  without  a  predominant  autumnal  pulse, 
and  after  September  the  numbers  fall  and  the  occurrences  become 
sporadic  until  the  following  April. 

The  pulses  of  this  species  are  listed  in  the  following  table,  which 
gives  their  locations  and  temperatures. 

Of  the  21  pulses  recorded,  18  are  within  the  periods  of  the  plant 
pulses  shown  in  Plates  I.  and  II.  Of  these  18  there  are  8  which 
coincide  with  these  plant  pulses,  9  which  follow  after  a  short  interval, 
and  1  which  shows  no  such  relation.  The  dependence  of  the  pulses 
of  Triarthra  upon  food  conditions  is  suggested.  The  pulses  of 
Triarthra  will  be  found  on  examination  of  Table  I.  to  coincide  in 
1898  in  the  main  with  those  of  the  total  Plointa. 

The  pulses  are  never  very  large,  and  the  evidences  of  reproduc- 
tion are  not  well  defined.  Attached  summer  eggs  attend  the  larger 
pulses,  and  free  winter  eggs  of  the  species  were  found  in  October- 
November  in  1898.  In  previous  years  free  or  attached  eggs  attended 
vernal  or  summer  pulses  at  times.  The  evidence  indicates  a  poly- 
cyclic  habit. 

*  See  Rousselet,  '02. 


218 


PULSES  OF  TRIARTHRA  LONGISETA. 


Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

7n° 

73° 

4  000 

"      27 

80° 

6,000 

31 

70° 

1,000 

Year 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

Date 

Temp. 

No. 

1895 

July   18 

80° 

19,080 

Aug.  21 

82° 

10,683 

Sept.  12 

79° 

2,336 

1897 

July  21 

81° 

49  ,  600 

Aug.  17 

79° 

9,600 

Sept.    7 

80° 

70,000 

1898 

July  26 

89° 

28,000 

Aug.  30 

83° 

6,400 

Sept.  27 

73° 

14,400 

This  is  an  exceedingly  variable  species.  It  varies  in  the  relative 
length  of  the  three  long  setae,  in  their  spinosity,  and  in  the  location 
of  the  posterior  one.  Many  of  the  individuals  in  our  waters  resemble 
the  form  described  by  Plate  ( '85)  as  T.  terminates.  The  long-spined 
form  described  by  Zacharias  ( '94)  as  var.  limnetica  is  also  abundant. 
It  is  doubtful  if  either  form  is  worthy  even  of  varietal  distinction. 

This  species  has  been  reported  only  from  Lake  Erie  and  the 
Illinois  River  in  this  country,  and  seems  to  be  rare  in  the  former. 
Weber  ('98)  finds  it  abundant  in  the  plankton  of  Lake  Leman; 
Burckhardt  ('00  and  'OOa)  reports  it  as  wide-spread  and  almost 
perennial  in  Swiss  lakes,  but  with  its  maximum  in  December- 
February,  and  slight  development  during  warmer  months.  Borge 
( '00)  finds  it  to  be  one  of  the  common  rotifers  in  the  summer  plank- 
ton in  Sweden ;  Marsson  ( '00)  reports  its  perennial  seasonal  range  in 
several  German  waters,  with  greater  numbers  during  the  warmer 
season.  Apstein  ('96)  gives  it  a  perennial  distribution  in  Lake 
Plon,  with  larger  numbers  in  June-November,  and  maximum  in 
June- July  or  August.  According  to  Seligo  ( '00)  the  species  is  per- 


219 

ennial  in  lakes  near  Danzig,  rivaling  Polyarthra  in  abundance,  and 
exhibiting  maxima  in  the  warmer  months  from  April  to  October. 

It  is  also  a  member  of  the  potamoplankton  of  European  streams. 
Skorikow  ('97)  finds  it  in  summer  months  in  the  Udy,  and  Zimmer 
('99)  reports  if  as  present  in  small  numbers  and  irregularly  in  the 
Oder  from  April  to  November.  Schorler  ('00)  finds  it  in  the  Elbe 
in  May-October  with  maxima  in  May  and  September,  and  Lauter- 
born  ( '98a)  includes  it  in  his  list  of  perennial  rotifers  in  the  plankton 
of  the  Rhine.  It  has  two  sexual  periods,  the  first  in  March-May 
and  the  second  in  July-October,  and  he  suggests  the  probability  of 
a  poly  cyclic  habit  in  some  waters. 

Trochosphczra  solstitialis  Thorpe  was  found  June  27,  July  2,  and 
August  15, in  1896 ;  in  1897, on  May  25  and  July  14-30.  Free  winter 
eggs  were  taken  August  15,  1896.  All  occurrences  were  above  66°. 
These  records  were  all  from  plankton  taken  in  mid-channel  of  the 
main  stream.  Trochosph&ra  was  found  in  greatest  abundance  at 
the  outlet  of  Flag  Lake  (Pt.  I.,  PI.  II.)  in  July,  reaching  9,664  per 
m.3  at  72°.  It  was  also  found  in  August  in  the  weedy  backwaters 
of  Dogfish  Lake.  Both  of  these  backwaters  connect  with  the  river 
(Pt.  I.,  PI.  II.)  below  the  point  at  which  our  collections  were  made. 
It  was  either  introduced  from  some  similar  backwater  higher  up  the 
stream  than  our  plankton  station,  or  developed  in  the  river  itself. 

SCIRTOPODA. 

This  order  is  represented  in  the  plankton  by  a  single  species, 
whose  discussion  will  suffice  for  the  order. 

Pedalion  mirum  Huds.  Average  number,  4,524.  This  is  a 
summer  planktont  of  somewhat  definite  temperature  limits.  The 
following  table  combined  with  the  data  in  Table  I.  will  suffice  to 
characterize  its  seasonal  fluctuations. 

Its  limitation  to  temperatures  above  60°,  indeed  almost  70°,  is 
apparent.  There  are  in  all  but  two  records  below  60°,  and  but  four 
below  70°.  It  is  a  typical  midsummer  planktont,  with  several 
recurrent  pulses  during  the  period  of  maximum  temperatures. 

The  location  of  these  pulses  with  reference  to  those  of  the 
chlorophyll-bearing  organisms  is  significant.  As  shown  in  Table  I., 
they  follow  immediately,  or  coincide  with,  those  of  the  synthetic 
organisms.  For  example,  the  apices  of  the  pulses  of  Mastigophora, 


220 


Year 

First  record 

First  maximum 

Date 

Temp. 

Date 

Temp. 

No. 

1894 

June  29 
July     6 
July   28 
July   21 
July   26 

83° 
80° 
80° 
84° 
89° 

2,592 
330,932 
20,000 
80  ,  000 
99,600 

1895 

1896  

May  25 
June  28 
June  21 

70° 

75° 
77° 

1897  

1898  

Year 

Second  maximum 

Last  record 

Date 

Temp. 

No. 

Date 

Temp. 

1894 

Sept.  17 
Oct.      2 
Sept.  16 
Sept.  14 
Nov.     1 

72° 
63° 
71° 

73° 
45° 

1895  

Aug.  21 
Aug.   15 
Aug.  17 
Aug.   16 

81° 
81° 
79° 

77° 

3,561 
77,600 
79,200 
22,400 

1896  

1897  

1898  

Bactilariacea,  and  Chlorophycecs  in  the  period  in  question  in  1898 
are  (PI.  II.)  July  19,  August  9,  August  30,  and  September  27.  The 
apices  of  the  Pedalion  pulses  are  July  26,  August  16,  and  September 
27,  the  last  coinciding  with  the  pulse  of  chlorophyll-bearing  organ- 
isms. In  1897,  the  intercalation  of  the  two  pulses  is  apparent,  and 
in  1896,  two  out  of  three  pulses  are  intercalated  and  a  third  is 
coincident.  As  will  be  seen  in  Table  I.,  these  pulses  of  1898  are 
approximately  coincident  in  many  cases  with  those  of  other  roti- 
fers— Synchceta,  Polyarthra,  Triarthra,  and  Brachionus.  The  sig- 
nificance of  this  intercalation  lies  probably  in  the  food  relations  of 
the  two  groups  of  organisms. 

Females  with  a  single  egg  attached  to  the  body  have  been  noted 
at  the  times  of  the  maxima  of  the  pulses,  or  immediately  thereafter, 


221 

in  five  instances.     On  the  pulse  of  July  26,  1898,  a  female  with  four 
male  eggs  was  found. 

This  species  was  not  reported  by  Apstein  ('96)  from  the  lakes 
of  Holstein,  but  was  found  by  Lauterborn  ( '98a)  in  the  Rhine  and 
its  backwaters.  Here  also  it  was  a  summer  form,  appearing  about 
the  middle  of  June,  with  a  maximum  in  August  or  September  and 
disappearing  late  in  October,  conditions  of  distribution  much  re- 
sembling those  in  the  Illinois.  It  is  regarded,  along  with  other 
summer  forms,  as  monocyclic.  The  appearance  in-  our  waters  of 
male  eggs  July  26,  at  the  height  of  the  first  pulse,  leads  to  the  in- 
ference that  there  may  be  several  cycles;  for  example,  three  in 
1898,  with  the  recurrent  pulses,  in  a  single  summer  season.  Weber 
( '98)  gives  it  as  a  summer  rotifer  in  Switzerland,  and  Skorikow  ( '97) 
finds  it  in  July-September  in  the  Udy  River,  in  Russia ;  but  it  is  not 
reported  from  the  Oder  by  Zimmer  ('99),  nor  from  the  Elbe  by 
Schorler  ('00).  Kellicott  ('97)  finds  it  in  Lake  Erie  in  small 
numbers  in  the  summer. 

In  addition  to  the  species  of  rotifers  noticed  above,  Hempel  ('99) 
has  reported  the  following  in  the  Illinois  River  or  its  backwaters: 
Flosculana  ornata  Ehrbg.,  Limnias  ceratophylli  Schrank,  Cephalosi- 
phon  limnias  Ehrbg.,  CEcistes  intermedius  Davis,  0.  mucicola  Kell., 
Pedetes  saltator  Gosse,  Furcularia  forficula  Ehrbg.,  F.  longiseta 
Ehrbg.,  Eosphora  aurita  Ehrbg.,  Diglena  grandis  Ehrbg.,  D.  catellina 
Ehrbg.,  D.  biraphis  Gosse,  Ccelopus  tenuior  Gosse,  Scaridium  longi- 
caudum  Ehrbg.,  Distyla  gissensis  Eckstein,  D.  okioensis  Herrick, 
D.  stokesi  Pell,  and  D.  hornemanni  Ehrbg. 

GASTROTRICHA. 

Ch&tonotus  sp.  occurred  singly  in  the  plankton  August  29,  1896, 
July  30,  1897,  and  February  15,  1898,  with  a  temperature  range  of 
32.5°  to  84°. 

ENTOMOSTRACA. 

Average  number,  47,042.  In  1897,  a  more  stable  year,  91,050; 
in  1896,  a  year  of  disturbed  hydrograph,  50,158;  in  1895,  in  more 
stable  conditions,  148,348.  The  Entomostraca  appear  in  every  collec- 
tion at  all  seasons  of  the  year.  The  decline  to  the  winter  mini- 


222 

mum  occurs  in  November-December.  Numbers  are  at  a  minimum 
(generally  less  than 5, 000 per  m.3)  in  midwinter  (January-February) ; 
rise  in  March  to  about  25,000  per  m.3;  and  attain  the  maximum  for 
the  year  in  a  vernal  pulse  of  200,000  to  1,500,000  in  April-May. 
Following  this,  there  is  frequently  a  second  pulse  of  large  proportions 
in  June,  which  in  1898  exceeds  (Table  I.)  that  of  May.  During  the 
remainder  of  the  year  there  is  usually  a  series  of  recurrent  pulses,  of 
declining  amplitude  in  1896  and  1898,  but  rising  to  unusual  heights 
(618,750  on  September  9)  in  the  stable  conditions  of  1897.  In 
the  main  the  pulses  of  Entomostraca  coincide  with  or  approximate 
to  the  location  of  those  of  the  other  organisms  of  the  plankton,  and 
often  show  correlations  in  amplitude. 

BRANCHIOPODA. 

Eubranchipus  serratus  Forbes.  Young  branchiopod  larvae 
questionably  referred  to  this  species  appeared  in  the  plankton 
in  January-March,  1899,  in  small  numbers  at  minimum  tempera- 
tures. 

CLADOCERA. 

Average  number,  6,068  per  m.3  In  1897  they  were  more  abun- 
dant, averaging  17,863  per  m.3  in  the  more  stable  conditions  of  that 
year.  In  1896,  a  year  of  recurrent  floods,  numbers  fell  to  7,7 19,  while 
in  1895,  a  year  of  low  water  in  spring,  when  many  of  the  Cladocera 
attain  their  maximum,  the  greatest  average,  31,937,  was  recorded. 
The  phenomenal  number  of  443,716  per  m.3  appeared  on  June  19  in 
the  stable  low  water  (1.80  ft.)  then  prevailing.  In  1894,  another 
year  of  low  levels,  the  annual  average  was  also  large  (23,952),  though 
probably  enhanced  by  the  fact  that  collections  were  not  made  in 
flood  waters  in  this  year. 

The  Cladocera  appear  in  all  but  10  of  the  182  collections  enu- 
merated, the  ten  exceptions  falling  in  November  (1),  January  (2), 
February  (6),  and  April  (1),  and  usually  in  flood  waters  or,  as 
in  1895,  in  stagnation  conditions  under  the  ice.  Although  the 
Cladocera  occur  in  all  months  of  the  year,  they  nevertheless,  as  a 
group,  exhibit  decided  temperature  adaptations,  as  appears  from 
the  fact  that  all  records  in  excess  of  4,000  per  m.3  fall  between  May  1 
and  September  1  with  but  6  exceptions, — 4  in  the  phenomenally 


223 


early  spring  of  1896,  and  2  in  the  delayed  high  temperature  of 
October,  1897. 

The  minimum  records  (less  than  500  per  m.3)  are  found  during 
minimum  temperatures.  The  numbers  increase  slightly  (generally 
less  than  2,000)  as  temperatures  rise  in  March- April,  rise  abruptly, 
as  they  approach  or  pass  70°,  to  a  vernal  maximum  in  May- June, 
and  decline  during  midsummer  excepting  when  unusual  pulses  of 
Moina  or  Diaphanosoma  raise  the  level  of  the  pulse  maxima  above 
25,000.  This  decline  continues  in  channel  plankton  through  the 
autumn  until  the  low  level  of  approximately  2,000  per  m.3,  at  the 
most,  is  again  attained  in  October,  and  falls  irregularly  to  500,  or 
less,  as  minimum  winter  temperatures  arrive  in  December.  Ex- 
ceptions appear  in  1897,  when  a  well-defined  autumnal  pulse  of 
large  amplitude  (193,500)  is  found  on  September  14,  and  is  followed 
by  others  of  declining  amplitudes  (137,600,  October  5;  5,520,  No- 
vember 15  ;  4,240,  December  14)  during  stable  autumnal  conditions. 

All  of  the  records  above  4,000  per  m.3,  with  one  exception,  are 
found  at  temperatures  above  45°,  and  all  in  excess  of  8,000,  with  4 
exceptions,  after  the  vernal  rise  in  temperature  passes  70°  in  April- 
May,  and  before  the  autumnal  decline  reaches  this  point  in  Septem- 
ber. The  Cladocera  are  thus  planktonts  of  the  warmer  channel- 
waters. 

The  relation  which  hydrographic  conditions  bear  to  the  seasonal 
occurrences  of  Cladocera  is  apparent  in  the  yearly  averages  above 
quoted,  and  appears  still  more  clearly  in  a  comparison  of  the 
cladoceran  population  and  movement  in  river  levels  in  July- 
December,  1897  and  1898,  as  given  below. 


Average  No. 
Cladocera 
per  m.3 

July 

August 

Sept. 

Oct. 

Nov. 

Dec. 

1897 

1898 

1897 

1898 

1897 

1898 

1897 

1898 

1897 

1898 

1897 

1898 

12720 

3050 

13960 

3756 

70675 

1700 

40350 

1615 

2532 

620 

1945 

236 

Total  movement 
in  river  levels, 
in  ft. 

5.2 

7.4 

2.6 

7.5 

0.6 

6.2 

0.6 

3.9 

2.2 

2.6 

0.5 

2.4 

224 

Hydrographic  changes  affect  the  Cladocera  by  increasing  the 
amount  of  silt  and  flocculent  debris  in  suspension,  which,  by  ad- 
herence to  the  swimming  antennas  and  flotation  processes  of  the 
animal,  tend  to  impede  its  movements  and  sink  it  to  the  bottom, 
where  it  is  removed  from  its  normal  feeding  area  and  readily  becomes 
the  prey  of  the  larger  organisms  of  the  bottom  fauna.  Barren  flood 
waters  also  tend  to  displace  and  wash  away  in  the  increased  current 
the  Cladocera  which  have  developed  in  the  stream,  and  to  afford 
both  less  food  and  less  time  for  their  further  development. 

The  occurrences  of  the  total  Cladocera  fall  into  the  type  of 
recurrent  pulses,  though  with  slightly  less  distinctness  than  in  the 
case  of  individual  species  of  the  group.  Such  pulses  can  be  traced 
in  all  seasons  in  which  records  were  made  at  short  intervals,  and 
suggestions  of  their  occurrence  appear  in  the  less  frequent  records 
of  other  seasons.  Thus  in  July-December,  1897,  (PI.  IV.),  there 
are  6  well-defined  pulses  culminating  at  intervals  of  3(1),  4(2),  5(1), 
and  6(1)  weeks.  In  1898  (Table  I.)  the  pulses  are  less  regular  in 
the  flood  waters  of  the  disturbed  year.  In  1896,  when  records  were 
frequent,  we  can  trace  pulses  in  March,  May,  June,  July,  August,  and 
September.  The  character  of  these  pulses  is  well  illustrated  in  the 
vernal  pulse  of  1898  (Table  I.  and  PI.  IV.),  culminating  June  7  at 
136,000.  The  species  which  share  in  this  pulse  are  Alona  affinis, 
A.  costata,  A.  quadrangularis ,  Bosmina  longirostris*,  Ceriodaphnia 
scitula*,  Chydorus  sphcericus* ,  Daphnia  hyalina*,  D.  cucullata*, 
Diaphanosoma  brachyurum,  Leptodora  hyalina,  Macrothrix  laticornis, 
Moina  micrura,  Pleuroxus  denticulate s,  Scapholeberis  mucronata,  and 
Simocephalus  serrulatus.  Of  these,  only  the  five  marked  by  the 
asterisk  occur  in  numbers  sufficient  by  our  methods  to  delineate  a 
pulse.  The  other  species  are  accordingly  of  little  consequence  in 
modifying  the  form  or  location  of  the  pulse.  The  June  volumetric 
pulse  (Part  I.,  PI.  XII.)  culminates  June  14  at  6.99  cm.3  per  m.3, 
though  the  record  for  June  7  is  also  high  (5.28).  The  cladoceran 
pulse  culminates  June  7  at  136,000.  On  this  same  day  four  of  the 
dominant  species  also  reach  their  culmination,  viz. :  Bosmina 
longirostris  (62,800),  Ceriodaphnia  scitula  (55,800),  Daphnia  cucul- 
lata (3,400),  and  D.  hyalina  (11,600),  the  remaining  2,400  being 
contributed  by  other  species.  Chydorus  spharicus,  which  appears 
this  spring  only  in  small  numbers,  attains  its  maximum  (7,880)  on 
May  24,  two  weeks  earlier,  though  the  record  for  May  31  is  also  high 


225 

(5,040),  indicating  a  probable  maximum  between  these  dates.  In 
other  seasons,  for  example  in  1896  and  1897,  the  maxima  of  this 
species  coincide  generally  with  those  of  other  Cladocera,  so  that  this 
divergence  seems  to  be  anomalous.  An  inspection  of  the  table  of 
records  for  1898  gives  a  remarkably  uniform  and  coincident  rise  and 
decline  of  the  pulses  of  the  several  species  which  constitute  this 
characteristic  vernal  pulse. 

No  effort  has  been  made  by  me  to  determine  the  total  cladoceran 
fauna  of  the  Illinois  River.  Only  those  species  are  here  given  which 
have  appeared  in  our  plankton  enumeration.  A  number  of  others 
are  known  to  occur  in  the  littoral  fauna,  and  a  few  scattering  indi- 
viduals found  in  the  plankton  were  not  identified. 

Of  the  25  forms  here  listed,  only  10 — named  in  the  sequence  of 
their  relative  numbers  as  shown  in  grand  totals — may  be  regarded 
as  typical  planktonts,  autolimnetic  in  channel  plankton-,  viz. :  Moina 
micrura,  Bosmina  longirostris,  Daphnia  cucullata  and  vars.  upicata 
and  kahlbergiensis,  D.  hyalina,  Ceriodaphnia  scitula,  Chydorus 
sphcericus,  Diaphanosoma  brachyurum,  and  Leptodora  hyalina.  Of 
the  ten,  the  last  named  and  the  varieties  of  D.  cucullata  appear  to 
be  of  little  quantitative  importance  in  the  channel  plankton,  though 
it  may  be  that  our  methods  of  collection  fail  adequately  to  represent 
Leptodora.  Of  the  remaining  15  species,  Alona  a~ffinis,  Ceriodaphnia 
reticulata  and  C.  rotunda,  Scapholeberis  mucronata,  and  the  two 
species  of  Simocephalus  are  the  only  adventitious  Cladocera  of 
quantitative  importance,  and  this  only  to  a  relatively  small  extent. 

DISCUSSION    OF    SPECIES    OF    CLADOCERA. 

Alona  affinis  Leydig. — Average  number,  36.  This  species  has 
a  well-defined  seasonal  distribution.  It  appears  in  autumn  in  the 
last  of  October,  as  temperatures  approach  40°,  and  remains  until 
the  end  of  June,  when  the  summer  maximum  of  80°  is  re-established. 
The  numbers  are  too  small  (Table  I.)  and  irregular  to  define  its 
seasonal  fluctuations,  though  there  are  suggestions  in  the  records 
of  late  autumnal  and  of  vernal  pulses.  Egg-bearing  females  were 
recorded  in  January-February  at  minimum  temperatures.  No  close 
dependence  on  hydrographic  fluctuations  is  apparent  to  account  for 
their  occurrence  in  the  plankton. 

Alona  costata  Sars. — Average  number,  11.  Only  a  few  scattered 
occurrences  of  small  numbers.  Earliest  autumnal  record,  Novem- 
ber 22,  at  40°;  latest  vernal,  May  24,  at  73°. 


226 

Alona  quadrangularis  O.  F.  Miill. — Average  number,  5.  A  few 
scattered  occurrences  in  March-May. 

Alona  spp. — It  is  probable  that  some  of  the  foregoing  species  of 
Alona  are  here  included.  There  are  16  occurrences,  scattered 
through  all  months  but  January,  April,  and  November,  with  no 
large  numbers  and  no  marked  seasonal  distribution. 

Bosmina  longirostris  O.  F.  Mull. — Average  number,  2,441,  of 
which  1,527  are  adult  females  without  large  embryos,  390  with 
them,  and  524  immature. 

I  include  in  this  species  B.  cornuta  Jurine,  for  I  am  unable  to  find 
any  constant  line  of  demarcation  between  these  forms.  The 
longirostris  form  is  the  dominant  one  in  the  channel  plankton,  the 
cornuta  form  being  relatively  rare. 

Bosmina  is  a  perennial  planktont  in  our  channel  plankton,  but 
occurs  in  small  numbers  only  in  October-May,  no  record  in  this 
period  with  the  exception  of  that  of  October  5,  1897  (20,400),  at 
71°,  exceeding  5,000  per  m.3,  and  most  of  them  falling  belowr  2,000. 
The  records  in  November-March,  with  the  exception  of  November- 
December,  1897,  all  fall  below  1,000  per  m.3  In  like  manner  the 
percentage  of  collections  containing  Bosmina  in  December-April  is 
lower  than  that  in  the  summer,  the  percentages  being  64,  16,  26,  47,. 
and  55  per  cent,  respectively  for  these  colder  months,  and  averaging 
82  per  cent,  for  the  rest  of  the  year.  The  percentage  of  occurrences 
in  October-November  remains  high  (82  and  81  per  cent.),  though 
the  numbers  per  m.3  fall  off  greatly. 

The  usual  seasonal  distribution  is  as  follows :  In  January-March 
the  occurrences  are  scattered  and  irregular  and  the  numbers  very 
small — less  than  500  per  m.3  Toward  the  close  of  April  the  vernal 
increase  makes  its  appearance,  continues  slowly  through  May,  rarely 
attaining  more  than  5,000  per  m.3,  and  at  the  end  of  this  month  or 
early  in  June  reaches  the  maximum  development  of  the  year  in  a 
vernal  pulse  of  40,320  (1896)  or  62,800  (1898)  per  m.3  From  this 
summit  there  is  an  abrupt  descent  in  a  period  of  exhaustion  to  a 
level  of  less  than  2,000  per  m.3  in  the  last  fortnight  of  June.  During 
the  remainder  of  the  year  there  appears  a  series  of  recurrent  pulses 
of  less  magnitude,  exceeding  10,000  per  m.3  in  but  three  instances. 
These  follow  at  intervals  of  four  to  six  weeks.  In  July-September 
the  amplitude  of  these  pulses  exceeds  in  all  cases  5,000  per  m.3  In 
October  (with  the  exception  of  1897,  when  temperatures  were  un- 


227 

usually  high),  they  decline  in  amplitude,  and  in  November-Decem- 
ber often  fail  to  appear  in  the  small  numbers  recorded.  In  1894, 
records  are  too  scanty  to  be  of  significance.  In  1895  there  are 
three  well-defined  pulses,  and  traces  of  a  fourth  in  August-Novem- 
ber. In  1896  there  are  five  in  May-September.  In  1897  there  are 
six  in  July-December,  data  during  the  remainder  of  the  year  being 
insufficient  to  define  the  pulses.  In  1898  the  vernal  pulse  in  June 
and  a  feeble  one  in  October  are  the  only  ones  which  appear.  The 
pulses  of  Bosmina  are  best  defined  in  the  stable  low  water  of  the 
last  six  months  of  1897.  During  that  period  they  closely  approxi- 
mate in  location  of  maxima  and  minima  the  quantitative  pulses 
and  those  of  the  chlorophyll-bearing  organisms  and  of  the  rotifers. 
(Compare  on  this  point  the  plates  for  1897  in  Part  I. — Kofoid,  '03— 
and  PI.  III.  and  IV.) .  The  slopes  of  the  pulses  indicate  that  Bosmina 
is  capable  of  very  rapid  multiplication;  and  their  coincidence  with 
other  pulses  just  noted,  taken  in  conjunction  with  the  fact  that 
males  and  ephippial  eggs  appear  but  rarely,  suggests  that  these 
pulses  of  Bosmina  are  immediately  dependent,  in  large  part,  upon 
fluctuations  in  the  food  supply  for  their  origin  and  for  the  varying 
courses  which  they  run. 

The  relations  of  Bosmina  to  temperature  appear  in  the  facts 
that  all  pulses  exceeding  5,000  per  m.3  in  amplitude  occur  at  tem- 
peratures above  70°,  that  the  vernal  rise  does  not  proceed  with  any 
rapidity  until  this  temperature  is  attained,  and  that  the  depressing 
effect  of  the  autumnal  decline  below  70°  is  at  once  apparent  in  the 
reduced  numbers  per  m.3  No  constant  relation  between  the  pulses 
of  Bosmina  and  the  midsummer  heat  pulses — such  as  appears  in 
the  records  of  Diaphanosoma — can  be  traced  in  the  occurrences  of 
Bosmina. 

An  inspection  of  the  accompanying  table,  in  which  the  mean 
monthly  Bosmina  population  per  m.3  of  channel  water  in  July-De- 
cember, 1897  and  1898,  is  given,  and  also  the  total  +  and  -  move- 
ment in  river  levels  for  these  months  in  each  year,  will  suggest  an 
intimate  connection  between  stability  of  hydrographic  conditions 
and  the  increase  of  Bosmina.  In  1897  the  total  movement  for  these 
months  is  from  five  sevenths  to  one  tenth  of  that  in  1898,  and  in 
every  instance  the  Bosmina  population  is  also  greater  by  from  7.5 
to  nearly  400-fold  in  1897,  the  more  stable  year.  The  means  of  the 
six  months  are  2.03  ft.  and  a  population  of  3,691  in  1897  to  5.3  ft.  and 


228 


BOSMINA  AND  HYDROGRAPHIC  FLUCTUATIONS.* 


Year 

July 

August 

September 

Total 
movement, 
in  feet 

Bosmina 
per  m.3 

Total   . 
movement, 
in  feet 

Bosmina 
per  m.3 

Total 
movement, 
in  feet 

Bosmina 
per  m.^ 

1897 

f  -3.9 

5 
I  +1.1 

6,213 

r  -2.6 

2.6\ 

(  +  o 

3,973 

r  -  -2 

.6 
I  +    -4 

3,022 

1898 

r  -6.9 

7U, 

140 

r  -3.3 

7.7 
I  +4.4 

10 

r-2.6 
6 

[  +3.4 

IS 

Year 

October 

November 

December 

Total 
movement, 
in  feet 

Bosmina 
per  m.3 

Total 
movement, 
in  feet 

Bosmina 
per  m.3 

Total 
movement, 
in  feet 

Bosmina 
per  m  3 

1897 

,(-•' 

I  +    .5 

5,875 

2.2\ 

[  +1.5 

1,680 

f  -    .6 

1.2 
I  +    .6 

1,585 

1898 

r  -1.1 

3.9 
1  +2.8 

780 

f  -    .6 
3.2 
I  +2.6 

32 

f  -2.8 
3.8] 
[  +1.0 

60 

*  +  =  rising  levels;    —  =  falling  levels. 

173  Bosmina  in  1898.  It  is  also  true  that  months  in  which  the 
disparity  in  stability  is  greatest  are  those  in  which  the  Bosmina 
ratios  are  greatest,  and  vice  versa.  It  seems  very  probable  that 
the  increased  current,  the  lessened  time  for  breeding,  and  the  greater 
burden  of  silt  in  flood  conditions,  especially  rising  waters,  do  not 
conduce  to  the  rapid  increase  of  Bosmina  in  channel  plankton. 

The  effect  of  the  high  temperatures  of  the  late  autumn  of  1897 
is  apparent  in  the  amplitude  of  the  October,  November,  and  De- 
cember pulses  (20,400,  3,440,  and  3,440,  respectively),  which  exceed 
those  of  all  other  years  at  this  season.  Temperature  thus  plays— 
perhaps  by  virtue  of  its  relation  to  the  food  supply — an  important 


229 


part  in  the  seasonal  delimitation  of  the  amplitude  of  Bosmina  pulses. 
The  Bosmina  population  in  the  plankton  consists  largely  of 
parthenogenetic  females.  Males  and  females  with  ephippial  eggs, 
were  recorded  only  in  October-December,  1897,  and  then  only  in 
small  numbers  and  isolated  occurrences.  Females  with  eggs  or 
embryos  and  the  free  young  were  found  at  all  seasons  of  the  year  and 
at  all  temperatures,  but  most  abundantly  at  the  time  of  the  pulses. 
Parasitized  or  fungused  individuals  are  also  found  occasionally  at 
these  seasons  of  greatest  numbers,  and  the  high  mortality  following 
a  pulse  is  evidenced  by  the  large  number  of  dead  occurring  in  the 
plankton.  The  proportions  of  females,  females  with  eggs  or  em- 
bryos, young,  and*  dead  during  the  May- June  pulse  of  1898,  may 
be  traced  in  the  following  records. 

BOSMINA  PER  M.3,  MAY-JUNE,  1898. 


Date 

Females 

Females 
with  eggs 

Young 

Total  living 

Dead 

Apr.  26  

800 

0 

0 

800 

0 

May    3  

1,600 

400 

800 

2,800 

0 

"      10   

1,600 

1,000 

1,000 

3,600 

400 

"      17  

1,300 

1,100 

1,100 

3,500 

100 

"      24   

3,280 

1,400 

1,240 

5,920 

920 

•    "      31  

25,120 

2,000 

6,800 

33,920 

1,280 

June    7   

38,800 

9,200 

14,800 

62  ,  800 

9,200 

"      14  

2,200 

3,000 

800 

6,000 

1,400 

"      21  

1,000 

500 

0 

1,500 

100 

"      28  

300 

200 

200 

700 

100 

Bosmina  longirostris  has  been  frequently  reported  in  the  plankton 
of  European  lakes.  Apstein  ('96)  finds  it  perennial  in  Plonersee 
with  larger  numbers  in  June-September  and  a  maximum  in  July. 
No  pulse-like  recurrence  is  noted,  parthenogenesis  prevails,  and 
males  and  ephippia  are  rare.  His  results,  save  in  the  matter  of 
pulses,  are  thus  in  general  accord  with  ours.  Stingelin  ('97)  notes 


230 

great  seasonal  polymorphism  in  B.  cornuta  near  Basel.  Zacharias 
('97a  and  '98b)  records  it  in  the  plankton  of  German  carp  ponds. 

Stenroos  ('97  and '98)  finds  it  in  waters  of  Finland  and  Karelia, 
where  the  cornuta  type  is  littoral,  and  a  limnetic  form,  distinguished 
by  him  as  forma  vernalis,  is  abundant  in  the  plankton  in  May.  Scour- 
field  ('98)  finds  it  common  in  the  waters  of  Epping  Forest,  where  it 
is  perennial,  males  and  ephippia  appearing  only  in  September- 
November.  According  to  Scott  ('99)  it  appears  at  various  seasons 
in  the  lochs  of  Scotland  in  both  the  littoral  and  limnetic  fauna. 
Burckhardt  ( 'OOa)  gives  an  extensive  revision  of  the  genus  Bosmina, 
and  includes  in  the  B.  longirostris  group  nine  other  so-called  species, 
among  which  are  B.  cornuta  Jur.  The  species  is  "pelagic  or  hemi- 
pelagic"  in  various  Swiss  lakes,  though  apparently  not  in  num- 
bers. The  genus  is  there  represented  in  the  plankton  princi- 
pally by  the  B.  coregoni  group.  Amberg  ( '00)  lists  it  from  Katzen- 
see,  near  Zurich,  as  a  perennial  planktont  with  large  numbers  in 
May,  August,  and  February,  but  gives  no  statistical  data.  Fuhr- 
mann  ('00)  finds  Bosmina  perennial  in  Neuenburgersee,  and  B. 
longirostris  with  a  maximum  in  May.  Marsson  ('00)  finds  B. 
"  longirostris-cornuta "  in  lakes  about  Berlin  throughout  the  year, 
with  larger  numbers  in  some  lakes  during  the  warmer  months  and 
in  others  in  November-December.  In  Barlewitzersee,  near  Danzig, 
Seligo  ('00)  reports  B.  cornuta  as  perennial,  with  maxima  in  June 
and  in  October-November,  the  latter  being  the  greater.  Larger 
numbers  appear  in  summer  than  in  winter.  Cohn  ('03),  in  waters 
near  Konigsberg,  finds  B.  longirostris  only  sparingly  present, 
appearing  in  May-September  with  a  maximum  in  July. 

In  European  streams,  also,  B.  longirostris  is  widely  distributed. 
Lauterborn  ( '94)  finds  it  abundant  in  the  winter  fauna  of  the  Rhine. 
He  also  states  that  it  is  not  acyclic  in  the  backwaters,  where  he  has 
found  in  three  successive  years  both  males  and  ephippia  in  May- June 
and  again  in  November.  There  is  thus  a  suggestion  of  a  vernal  and 
an  autumnal  pulse  in  these  waters.  Zimmer  ( '99)  finds  it  through- 
out the  whole  year  in  the  Oder.  Schorler  ( '00)  reports  it  from  the 
Elbe  at  Dresden  in  May-October,  with  larger  numbers  in  May- June 
and  September,  while  Fric  and  Vavra  ('01)  find  it  in  the  same 
stream  near  Podiebrad.  They  state  that  B.  cornuta  is  found  in 
great  numbers  in  1  m.-  surf  ace  in  summer  months,  and  B.  longiros- 
tris sparingly  in  the  littoral  fauna.  Steuer  ('01)  finds  B, '  'longirostris- 


231 

cornuta"  m  the  backwaters  of  the  Danube  at  Vienna  in  April-January. 
It  exhibits  a  distinct  seasonal  polymorphism,  with  a  large  winter 
form  and  a  smaller  summer  one.  Data  as  to  relative  numbers 
during  the  year  are  not  given.  Skorikow  ('02),  in  reviewing  the 
investigations  on  the  plankton  of  Russian  waters,  reports  B.  cornuta 
from  the  summer  plankton  of  several  streams,  but  expresses  doubts 
as  to  whether  "sie  als  autopotamische  Planktonorganismen  anzu- 
sehen  sind  oder  nicht."  Meissner  ('03)  finds  B.  cornuta  generally 
in  the  Volga  and  its  adjacent  waters  in  the  summer  plankton,  with 
largest  numbers  in  August;  and  Zykoff  ('03)  reports  it  in  small 
numbers  from  the  same  stream  in  May- July.  It  is  not  listed  by 
Volk  ('03)  in  the  Elbe  at  Hamburg. 

B.  longirostris  occurs  generally  in  American  waters,  though 
apparently,  often  in  small  numbers.  Thus  Forbes  ('82  and  '90) 
reports  it  in  the  plankton  of  Lake  Michigan  and  Lake  Superior,  and 
it  appears  generally  in  lists  of  Cladocera  from  many  widely  separated 
smaller  bodies  of  water  in  this  country.  Birge  ('95  and  '97)  finds 
only  a  few  Bosmina  (species  not  stated)  in  Lake  Mendota,  but 
Marsh  ('97)  reports  it  (species  not  given)  as  perennial  in  Green 
Lake,  with  a  maximum  in  November.  His  records  have  also  a 
suggestion  of  an  earlier  pulse,  in  June,  in  which  month  there  is  a 
sudden  rise  from  a  previous  minimum. 

This  partial  survey  of  the  literature  of  the  records  of  Bosmina 
in  the  plankton  shows  its  wide  distribution,  suggests  the  probability 
of  great  variation,  necessitating  caution  in  the  description  of  new 
species  in  this  genus,  and  indicates  a  wide  diversity  in  its  seasonal 
career  even  in  waters  with  somewhat  closely  similar  environmental 
conditions. 

Ceriodaphnia  me  gaps  Sars  was  found  singly  but  once — July  25, 
1896,  at  80°. 

Ceriodaphnia  reticulata  Jurine  was  found  in  the  plankton  occa- 
sionally, and  always  in  small  numbers,  in  April-September.  All 
occurrences  appear  at  temperatures  above  66°,  and  the  earliest  is 
on  April  1 7 ,  and  the  latest  is  September  2 1 .  Females  with  summer 
eggs  were  found  in  June-September. 

Ceriodaphnia  rotunda  Straus  was  recorded  in  1894-1895,  but  not' 
thereafter.  Its  identification  is  somewhat  questionable,  and  if 
correct,  this  is  apparently  the  first  record  of  this  species  in  North 
American  waters,  unless  it  should  appear  that  C.  alabamensis 


232 

Herrick  or  C.  acanthinus  Ross,  which  appear  to  resemble  C.  rotunda 
in  some  particulars,  should  be  included  here  as  forms  or  synonyms. 
The  genus  is  sadly  in  need  of  revision. 

The  forms  referred  to  C.  rotunda  were  found  in  August,  1894,  and 
July-August,  1895,  16,536  per  m.3  appearing  in  the  plankton  on 
July  18  of  the  latter  year. 

Ceriodaphnia  scitula  Herrick. — Average  number,  1,539.  This 
species  is  closely  related  to  the  European  C.  quadrangula  O.  F.  Mull., 
if,  indeed,  it  is  not  identical  with  it.  It  is  not  impossible  that  it  is 
the  form  imperfectly  described  by  Say  ('18)  as  Daphnia  angulata. 
In  the  absence  of  a  critical  monograph  of  the  genus  I  use  the  name 
applied  in  current  American  literature  to  this  form. 

This  is  the  most  abundant  species  of  the  genus  in  our  waters, 
outnumbering  all  others  by  over  sixfold  in  the  totals  of  our  records. 
It  is  also  one  of  the  most  important  members  of  the  Entomostraca 
in  the  channel  plankton  (total  of  all  records,  156,119),  being  ex- 
ceeded in  numbers  only  by  Moina  micrura  (1,121,808),  Bosmina 
longirostris  (381,598),  Daphnia  cucullata  (237,444),  and  D.  hyalina 
(231,746). 

It  occurs  in  all  months  of  the  year  except  January  and  February, 
but  in  larger  numbers  and  in  more  of  the  collections  in  May-Septem- 
ber. Thus  less  than  6  per  cent,  (reduced  to  2  per  cent,  if  one  col- 
lection in  the  warm  autumn  of  1897  is  omitted)  of  the  individuals 
and  only  20  of  the  79  occurrences  are  found  outside  of  the  May- 
September  period.  Ceriodaphnia  scitula  is  accordingly  a  summer 
planktont  in  channel  waters.  It  is  found  in  each  year,  though  in 
varying  numbers  according  to  hydrographic  and  other  conditions. 
Thus  in  1898  the  vernal  pulse  in  June  attains  the  unsurpassed 
amplitude  of  55,800  per  m.3,  but  declines  in  a  fortnight  and  makes 
no  recovery  during  the  disturbed  hydrographic  conditions  of  the 
summer.  In  1897,  on  the  other  hand,  our  records  were  too  meager 
to  delineate  fully  the  vernal  pulse,  and  in  the  stable  conditions  of 
the  summer  and  autumn  the  species  continued  in  numbers  whose 
totals  exceed  those  of  1898  by  81 -fold.  Similarly  in  1896  the  more 
gradual  changes  in  levels  which  attended  the  floods  of  that  year 
permitted  a  considerable  development  of  Ceriodaphnia  throughout 
the  summer.  Stable  hydrographic  conditions  thus  conduce  to 
increase  in  Ceriodaphnia.  The  relations  which  I  have  shown  to 
exist  between  Bosmina  and  movement  in  river  levels  (see  table  on 


233 

page  228)  exist  also  in  the  case  of  Ceriodaphnia  and  in  much  the 
same  form. 

The  relation  of  temperature  to  Ceriodaphnia  is  evident  in  its 
seasonal  distribution.  It  does  not  advance  rapidly  in  its  vernal 
increase  until  after  the  water  warms  to  70°,  and  drops  suddenly  in 
numbers  when  the  autumnal  decline  passes  this  point.  Moreover, 
seasonal  variations  in  temperature  are  accompanied  by  correspond- 
ing shiftings  of  the  pulses  of  Ceriodaphnia.  Thus  in  1898  the  water 
did  not  reach  70°  until  about  May  20,  reaching  73°  on  May  24,  and 
the  vernal  pulse  of  Ceriodaphnia  began  at  once  its  rise  to  the  maxi- 
mum of  June  7.  In  1896  spring  was  early,  72°  being  recorded  in 
surface  waters  on  April  24,  and  we  find  a  vernal  pulse  rising  to  a 
maximum  on  May  8.  So  also  in  1897,  when  high  temperatures 
continued  into  the  autumn,  the  decline  passing  71°  on  October  5, 
instead  of  in  the  first  half  of  September  as  in  other  years,  we  find 
the  pulses  of  Ceriodaphnia  extending  into  October  with  unusual 
amplitude,  reaching  5,200  per  m.3  October  5,  while  the  highest 
record  in  this  month,  or  later,  in  other  years  was  280  per  m.3  Tem- 
perature rather  than  season  is  thus  the  dominant  factor  in  the 
seasonal  curve  of  occurrence  of  Ceriodaphnia. 

The  form  of  this  seasonal  curve  is  typically  that  of  a  series  of 
recurrent  pulses  of  varying  magnitude  tending  to  reach  the  maxi- 
mum height  in  the  vernal  pulse  of  May- June,  attaining  often  lower 
levels  in  July  and  rising  again  in  August-September,  and  falling  to 
a  minimum,  or  even  to  disappearance,  in  October.  These  later 
pulses  do  not  appear  in  the  disturbed  hydrographic  conditions  of 
1898  (Table  I.),  but  are  clearly  delineated  in  the  summer  records 
of  other  years,  especially  in  the  stable  conditions  of  1897,  where 
well-defined  pulses  appear  in  July,  August,  September,  and  October, 
at  intervals  of  approximately  four  weeks,  culminating  July  14, 
August  10,  September  14,  and  October  5.  Their  maxima  attain 
respectively  5,600,  2,720,  6,000,  and  5,200  perm.3,  and  the  pulses  are 
delimited  in  each  case  by  minima  of  less  than  500  per  m.3  They 
tend  to  coincide  with  those  of  other  Entomostraca  and  to  approach 
those  of  the  Rotifer  a. 

The  Ceriodaphnia  population  in  channel  waters  is  almost  ex- 
clusively made  up  of  parthenogenetic  females.  Males  were  not 
recorded  at  any  time,  though  females  with  ephippial  eggs  appeared 
after  the  October  pulse  of  1897  and  the  vernal  one  of  1898. 


234 

Ceriodaphnia  scitula  appears  but  once  in  the  records  of  European 
plankton,  Scourfield  ( '98)  finding  it  in  the  waters  of  Epping  Forest 
in  September.  The  closely  related  C.  quadrangula  as  well  as  the 
other  species  have  been  frequently  recorded  by  European  investi- 
gators both  in  the  littoral  and  the  limnetic  fauna,  but  they  appear 
to  be  less  generally  found  there  than  the  other  dominant  Cladocera 
of  our  waters. 

It  does  not  appear  in  the  plankton  of  our  Great  Lakes  (Forbes  '82 
and  '90,  Birge  '95),  or  in  that  of  Lake  Mendota  (Birge  '95  and  '97), 
or  .Green  Lake  (Marsh  '97),  but  Herrick  ('84)  reports  it  as  the  most 
abundant  species  in  Minnesota,  and  Fordyce  ('00)  finds  it  in 
Nebraska  in  shallow  waters.  A  revision  of  the  genus  is  needed 
before  the  seasonal  distribution  of  the  various  species  can  be  worked 
out  on  a  basis  that  will  make  satisfactory  discussions  of  the  literature 
possible. 

Chydorus  sphcericus  O.  F.  Mull. — Average  number,  422,  of  which 
26  are  egg-bearing  females,  and  6  are  immature,  the  remainder,  390, 
being  females  in  which  the  ova  were  not  prominent. 

The  identification  of  species  of  Chydorus  is  attended  by  consider- 
able uncertainty.  Comparison  with  named  specimens  from  Europe 
supplied  by  Prof.  G.  O.  Sars,  leaves  no  doubt  that  C.  sph&ricus  is 
common  in  our  waters,  and  it  is  apparently  the  dominant  species. 
It  is  probable  that  several  other  species,  as,  for  example,  C.  globosus 
Baird  and  C.  c&latus  Schoedler,  occur  sparingly  in  our  waters  and 
have  been  included  with  C.  sphczricus  in  my  enumerations.  The 
difficulties  which  attend  the  attempt  to  assign  every  individual  to 
one  of  the  several  species  of  Chydorus  can  be  appreciated  only  by 
one  who  makes  the  effort.  The  problem  of  their  specific  validity 
should  be  solved  by  a  statistical  analysis  of  the  range  of  varia- 
tion. 

The  seasonal  distribution  of  Chydorus  sphcericus  in  channel 
waters  is  in  its  general  outlines  very  characteristic  and  well  defined. 
The  following  table,  which  gives  the  average  number  of  Chydorus 
per  m.3  for  each  month  of  our  collections,  shows  clearly  that  it  is  a 
vernal  planktont,  and  that  there  is  a  slight  tendency  toward  an 
autumnal  pulse  in  September,  when  vernal  temperatures  return. 
The  number  for  November  (222)  would  probably  be  considerably 
reduced  if  more  than  one  collection  had  been  taken  in  that  month 
in  1896.  Omitting  this  year,  the  average  for  November  falls  to 


235 


78,  and  a  secondary,  hiemal  rise  becomes  apparent  in  December. 
This  December  pulse  of  Chydorus  is  one  of  the  elements  in  the 
upward  movement  of  production  in  this  month  (see  Part  I.),  and 
fuller  data  may  serve  to  connect  it  fully  with  the  September-October 
pulse,  especially  in  more  stable  conditions.  Both  of  these  autumnal- 
hiemal  movements  have  less  than  one  tenth  of  the  development  that 
the  vernal  pulse  exhibits. 

The  number  and  percentage  of  occurrences  also  confirm  the 
conclusions  drawn  from  the  numbers  per  m.3  Percentages  run 
higher  in  the  spring,  in  March-May,  and  in  September-October 
and  in  December,  and  lower  in  June- August,  November,  and 
January-February.  Chydorus  occurred  in  all  March  collections, 
and  in  only  one  third  of  the  August  collections. 

The  analysis  of  the  data  in  this  table  indicates  the  presence  of 
Chydorus  in  the  plankton  practically  throughout  the  whole  year  in 
the  whole  seasonal  range  in  temperatures,  with  the  larger  develop- 
ments following  shortly  after  the  thermograph  passes  the  yearly 
mean  (57°  average  of  monthly  means  of  surface  waters)  in  vernal 
rise  and  autumnal  decline,  the  maximum  development  in  April-May 

SEASONAL  DISTRIBUTION  OF  CHYDORUS.     AVERAGE  NUMBER  PER  M.3 


Year 

Jan. 

Feb. 

March 

April 

May 

June 

1  804. 

234 

1895 

11 

2  044 

0 

1896  

304 

167 

1,682 

10,271 

5,701 

448 

1897 

20 

540 

320 

32  800 

900 

1898    

160 

0 

256 

300 

3,364 

356 

1  S99 

36 

65 

193 

Average  

167 

53 

668 

3,235 

13,955 

388 

No  of  occurrences  

9 

6 

15 

9 

9 

10 

Percentage     of     occur- 
rences   

75 

40 

100 

82 

90 

72 

236 


SEASONAL  DISTRIBUTION  OF  CHYDORUS.     AVERAGE  NUMBER  PER  M.3 — continued. 


Year 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

1894  

95 

0 

461 

100 

16 

56 

1895  

91 

103 

164 

38 

203 

448 

1896  

64 

104 

78 

160 

800 

277 

1897  

213 

40 

407 

650 

64 

115 

1898  

50 

0 

30 

60 

28 

172 

1899 

Average  

103 

49 

228 

202 

222 

214 

No.  of  occurrences  

11 

7 

13 

12 

10 

14 

Percentage  of  occur- 
rences   

61 

33 

81 

71 

63 

82 

occurring  in  average  temperatures,  for  these  months,  of  60.5°  and 
68.3°,  while  the  minor  autumnal  development  appears  in  September- 
October  at  74.2°  and  57.6°  respectively,  and  the  December  pulse,  if 
indeed  it  be  a  separate  and  independent  pulse,  is  at  the  low  tempera- 
ture of  35.2°.  The  December  movement  may  be  simply  the  result 
of  the  more  stable  conditions  which  attend  the  appearance  of  the 
ice-sheet  on  the  approach  of  winter. 

An  analysis  of  the  course  of  the  seasonal  distribution  of  Chydorus 
in  channel  waters,  as  given  in  Table  I.  and  in  statistics  of  other 
years,  indicates  the  following  seasonal  regimen.  In  January-Feb- 
ruary, at  minimum  temperatures,  the  occurrences  are  irregular 
(75  and  40  per  cent.)  and  the  numbers  small  (average,  167  and  53 
per  m.3),  while  in  March,  with  rising  temperatures,  occurrences  are 
more  numerous  (100  per  cent.)  and  numbers  rise  to  668  per  m.3  In 
April-May  a  high  percentage  of  occurrences  (82  and  90  per  cent.) 
continues,  and  they  mount  rapidly  to  the  maximum  record  of  the 
year,  which  in  our  statistics  varies  from  4,088  in  1895  to  32,800  in 
1897.  This  vernal  pulse  reaches  its  maximum  in  our  records  on 
April  29  in  1895,  at  64°,  and  in  1896  on  the  same  day,  at  70°;  on 


237 

May  25  in  1897,  at  66. 3°;  and  on  May  24,  in  1895,  at  73°.  From  this 
maximum  the  pulse  declines  abruptly  in  a  fortnight  to  a  midsummer 
minimum  during  maximum  temperatures,  which  continues  until 
September.  During  this  period  the  numbers  are  small,  rarely  rising 
above  400  per  m.3  (average,  388,  103,  and  49),  and  the  occurrences 
are  also  less  numerous  (72,  61,  and  33  per  cent.).  With  the  decline 
of  temperatures  which  begins  in  September  the  percentage  of  occur- 
rences mounts  to  81,  and  the  average  per  m.3  to  228,  and  remains 
near  this  level  during  the  remainder  of  the  year. 

An  analysis  of  the  full  statistical  data,  of  which  the  records  for 
1898  are  fairly  typical,  confirms  the  conclusions  drawn  from  these 
averages.  Chydorus  in  channel  waters  is  monocyclic,  with  a  well- 
defined  vernal  pulse  in  March- June  which  includes  95  per  cent,  of 
the  total  annual  Chydorus  population.  There  are  suggestions  of  an 
autumnal  pulse,  but  the  data  are  not  sufficient  to  delimit  it.  There 
is  no  satisfactory  evidence  that  there  are  recurrent  cycles  or  pulses 
at  briefer  intervals  during  the  year. 

The  dominating  effect  of  temperature  as  a  regulating  factor  in 
delimiting  the  seasonal  distribution  of  Chydorus  is  very  evident. 
This,  in  addition  to  its  appearance  in  the  annual  curve  of  occurrences, 
is  also  exhibited  most  clearly  in  a  comparison  of  the  vernal  pulses 
in  the  two  years  of  fullest  representation  in  our  records,  1896  and 
1898.  The  following  table  gives  the  data  of  dates,  temperatures 
of  surface  waters,  and  numbers  of  Chydorus. 

From  these  facts  it  appears  that  the  late  spring  of  1898  delayed 
the  vernal  pulse  of  Chydorus,  and  that  the  early  spring  of  1896 
accelerated  it  in  that  year  so  that  their  apices  (April  29  and  May  24) 
are  four  weeks  removed  from  each  other  in  seasonal  location.  In 
both  years  the  rapid  rise  in  the  pulse  appears  after  60°  is  passed,  the 
culmination  occurs  at  about  70°,  and  the  decline,  in  temperatures 
above  70°. 

Egg-bearing  females  were  more  abundant  during  the  rise  of  the 
pulse,  and  less  numerous  during  its  decline.  Evidence  of  great 
mortality  during  the  decline  of  the  pulses  is  to  be  found  in  great 
increase  in  the  relative  numbers  of  empty  carapaces.  Thus,  during 
the  decline  of  the  vernal  pulse  in  1896  there  were  on  the  day  of 
culmination,  April  29,  2,780  dead  to  18,904  living,  on  May  1,  3,570 
to  14,875,  and  on  May  8,  1,578  to  6,706.  From  14  to  24  per  cent,  of 
the  Chydorus  population  had  thus  recently  perished.  Parasitized 


238 


1896 

1898 

Date 

Tempera- 
ture 

No.  of 
Chydorus 

Date 

Tempera- 
ture 

No.  of 
Chydorus 

Mar.  17  

42° 

256 

Mar    15 

46° 

440 

"     24  

40  7° 

610 

"      22 

51° 

480 

"     30  

48.1° 

6  405 

"      29 

49  5° 

240 

Apr.  10  

46  4° 

1   666 

Apr      5 

48   3° 

200 

"     17  

66  3° 

4  515 

12 

52° 

200 

"      24 

72° 

1  S    000 

"      1  0 

56° 

"      29  

68° 

18  904 

"      26 

57° 

800 

May     1 

68   8° 

14  875 

60° 

8  

76° 

6   706 

"      10 

62° 

600 

"      18  

71   2° 

1    143 

"      17 

64° 

3   300     . 

"     25  

75  3° 

80 

"      24 

73° 

7   880 

June    6  

79° 

320 

"     31  
June    7 

70° 
78° 

5,040 
600 

"      11  

73° 

320 

14 

82   3° 

200 

and  fungused  individuals  were  also  noted  in  these  periods  of  decline. 
Males  were  recorded  in  September,  December,  and  February. 

Chydorus  is  not  given  as  a  constituent  of  the  plankton  of  Nor- 
wegian lakes  by  Huitfeldt-Kaas  ( '98)  or  of  Swiss  lakes  by  Fuhrmann 
('00),  Amberg  ('00),  or  Burckhardt  ('00  and  'OOa).  Its  absence 
from  these  cooler  waters  stands  in  sharp  contrast  with  its  abundance 
in  warm  and  shallow  European  lakes.  It  is  reported  as  abundant 
in  Chroococcacea-rich  lakes  of  North  Germany  by  Apstein  ('96), 
where  it  is  acyclic,  with  larger  development  in  April-October,  and 
maximum  in  August  or  in  May- June.  According  to  Weismann 
(79)  Chydorus  in  some  waters  is  polycyclic.  It  is  also  reported  by 
Zacharias  ('97a  and  '98b)  from  the  pond  fauna  of  Trachenberg  and 
many  other  German  localities,  where  it  forms  '"em  notorisches 


Mitglied  des  Teichplanktons."  He  also  lists  it  ('98b)-  from  some 
German  streams.  Marsson  ( '00)  found  it  in  some  waters  near  Berlin 
in  April- August,  noting  a  great  abundance  in  one  instance  in  May. 
Seligo  ('00)  gives  a  few  statistical  data  indicating  the  occurrence 
of  Chydorus  in  the  plankton  of  Hintersee  near  Danzig  in  April- 
December,  with  a  maximum  in  August  and  a  secondary  one  in 
October.  It  was,  however,  sparingly  present  in  adjacent  waters. 
Cohn  ('03)  finds  a  like  irregularity  in  its  occurrence  in  waters  near 
Konigsberg. 

Stenroos  ('97)  finds  it  to  be  one  of  the  most  abundant  Entomos- 
traca  in  the  waters  of  northern  Russia  and  ( '98)  a  littoral  and  bottom 
species  near  Helsingfors.  Scourfield  ('98)  finds  it  to  be  one  of  the 
most  abundant  Cladocera  in  the  waters  of  Epping  Forest,  occurring 
from  March  to  December,  with  maxima  of  sexual  reproduction  in 
April  and  November.  Scott  ('99)  reports  it  as  abundant  in  the 
littoral  fauna  of  Scottish  waters,  but  rare  in  tow-net  collections  in 
open  water. 

It  also  occurs  in  the  potamoplankton  of  European  streams, 
Zacharias  ('98b)  listing  it  from  a  few  minor  streams,  but  without 
seasonal,  statistical,  or  temperature  data.  It  was  not  separately 
listed  by  Skorikow  ('97)  in  the  summer  plankton  of  the  Udy  at 
Charkow,  or  by  Lauterborn  ('94)  in  the  winter  plankton  of  the 
Rhine.  Zimmer  ( '99)  found  it  from  February  to  July  in  the  Oder, 
and  Schorler  ( '00)  finds  it  abundant  in  the  plankton  of  the  Elbe  in 
April.  Steuer  ('01)  finds  it  at  all  seasons  in  the  backwaters  of  the 
Danube  at  Vienna,  and  in  the  plankton  from  March  to  November 
"oft  in  grossern  Mengen,"  but  gives  no  statistics  of  its  seasonal 
distribution.  Fric  and  Vavra  {'01)  find  it  in  the  channel  and 
backwaters  of  the  Elbe  near  Podiebrad,  but  more'  abundant  in  the 
littoral  fauna,  though  no  quantitative  or  statistical  data  of  its 
occurrence  are  given.  Zykoff  ('03)  reports  it  as  present  in  the 
plankton  of  the  Volga  at  all  times  in  small  numbers,  and  suggests 
a  predominance  in  May- July.  Meissner  ( '03)  also  reports  it  for  the 
Volga,  but  states  that  it  is  predominantly  a  member  of  the  littoral 
fauna  though  present  in  the  plankton  of  the  stream  in  restricted 
numbers.  No  statistical  data  are  given  by  him.  Volk  ('03)  reports 
it  in  the  Elbe  at  Hamburg,  but  without  any  details. 

This  species  is  reported  generally  from  American  waters.  Forbes 
C90)  reports  it  in  the  summer  plankton  of  Lakes  Superior  and 


240 

Michigamme  in  small  numbers,  and  ( '93)  in  that  of  the  Alpine  waters 
of  Wyoming  and  Montana,  where  it  is,  however,  more  abundant  in 
smaller  pools.  Birge  ('94)  finds  it  generally  distributed  in  collec- 
tions, including  plankton,  in  Lake  St.  Clair  and  ('97)  a  member  of 
the  plankton  of  Lake  Mendota,  where  its  abundance  is  dependent 
on  the  supply  of  Anab&na.  Its  maximum — only  a  single  well-defined 
one  occurring  in  each  year — was  found  in  July-October.  Birge 
regards  it  as  an  accidental  member  of  the  limnetic  fauna,  maintained 
there  as  long  as  suitable  food  is  present.  Its  mode  of  occurrence 
does  not,  however,  differ  from  that  of  typical  plankton  organisms, 
which  would  doubtless  likewise  disappear  from  the  plankton  if  their 
food  should  be  lacking. 

It  is  noteworthy  in  this  connection  that  it  was  only  sparingly 
present  in  the  channel  of  the  Illinois  in  the  midsummer-autumn 
plankton,  when — as,  for  example,  in  1897 — Anab&na  and  its  allies 
were  abundant.  It  seems  "not  improbable  that  temperature  even 
more  than  food  is  an  important  factor  in  controlling  its  seasonal 
and  local  distribution.  It  is  unquestionably  a  member  of  the 
plankton  in  our  waters,  though  also  abundant  here,  as  elsewhere,  in 
the  littoral  fauna.  In  our  locality  in  channel  plankton  it  shows 
distinctly  seasonal  limitations  which  suggest  the  operation  of  tem- 
perature rather  than  food.  Its  occurrence  in  large  numbers  in 
Wisconsin  lakes  in  midsummer  and  its  absence  in  the  Illinois  at 
that  time  may  also  be  correlated  in  part  with  the  contrasted  tem- 
perature conditions  in  the  two  localities.  Its  occurrence  in  our 
littoral  fauna  may  also  in  part  be  due  to  the  lower  temperatures 
consequent  upon  spring-fed  areas  and  the  shade  of  aquatic  vegeta- 
tion. Chydorus  is  one  of  those  organisms  capable  of  both  the  littoral 
and  limnetic  habit  under  suitable  conditions  of  food  and  temperature. 
In  our  waters,  at  least, — and,  as  it  seems  from  the  data  of  distribu- 
tion, elsewhere, — temperature,  rather  than  food  directly,  appears 
to  be  the  factor  controlling  the  occurrence  of  Chydorus  in  the 
plankton. 

Daphnia  cucullata  G.  O.  Sars. — Average  number,  181.  In  1897, 
very  much  greater, — 5,483  per  m.3 

For  the  reasons  given  by  Burckhardt  ('00)  I  use  Sars's  name 
cucullata  rather  than  jardinei  of  Richard  to  designate  those  forms 
of  the  subgenus  Hyalodaphnia  in  our  plankton.  In  channel  waters 
this  species  varies  considerably,  but  not  to  the  extent  that  it  does 


241 

where  its  numbers  are  greater.  The  forms  known  as  apicata 
Kurz  and  kahlbergiensis  Schoed.  appear  in  small  numbers  in  some 
years. 

This  species  appears  in  our  collections  in  April-December  only, 
with  the  exception  of  one  occurrence  in  January  and  two  in  March. 
Its  occurrences  and  numbers  vary  greatly  in  different  years.  In 
1894-95  its  numbers  were  small  and  occurrences  scattering,  it  being 
most  abundant  in  November-December.  In  1896  there  was  a 
large  vernal  development  in  April- June,  and  a  series  of  diminishing 
pulses  in  July-September.  In  1897  no  vernal  development  appeared 
in  our  scattered  collections,  but  in  the  stable  conditions  of  late 
summer  and  autumn  occurred  the  largest  development  recorded  in 
any  year,  with  a  maximum  record  of  72,760  per  m.3  on  October  5.  In 
1898  there  was  a  small  vernal  development  (3,400)  in  May- June 
and  a  still  smaller  one  (600)  in  October.  A  well-defined  seasonal 
routine  is  thus  not  demonstrable  from  our  data,  though  the  fact 
that  both  the  percentage  of  occurrences  and  the  numbers  are  highest 
in  May- June  and  September-October  suggests  a  tendency  toward 
vernal  and  autumnal  pulses  separated  by  a  period  of  less  develop- 
ment in  midsummer  and  of  autumnal  decline  followed  by  a  period 
of  almost  complete  extinction  in  midwinter. 

The  statistics  of  the  D.  cucullata  population  in  all  years  in  which 
weekly  collections  were  made,  exhibit  very  clearly  the  phenomenon 
of  recurrent  pulses  of  3  to  5  weeks '  duration,  with  maxima  of  varying 
amplitude  and  minima  of  less  than  400  per  m.3  in  all  cases  but  those 
which  mark  the  September  pulse  of  1897.  There  are  in  1896  pulses 
culminating  April  24  (2,544  per  m.3),  May  8  (11,965),  June  11 
(12,000),  July  18  (1,040),  August  8  (800),  and  September  16  (507). 
In  1897,  vernal  records  are  incomplete.  Pulses  appear  July  14  (800), 
August  17  (1,680),  September  14  (57,000),  October  5  (72,760),  and 
November  15  (2,040).  These  pulses  coincide  exactly  or  approxi- 
mately with  those  of  the  other  Entomostraca  which  exhibit  the  same 
phenomenon,  and  approximate  also  those  of  the  Rotifera.  A  typical 
pulse,  that  of  October,  1897,  is  shown  in  the  following  table.  It 
is  a  noticeable  fact  that  the  proportion  of  immature  forms  is  often 
greater  at  and  after  the  period  of  maximum  development  than  at 
other  times,  as  appears  in  the  table. 

The  relations  of  temperature  to  the  development  of  D.  cucullata 
in  channel  waters  appear  in  the  fact  that  all  occurrences  in  excess  of 


242 


Date 

Females 

Females 
with  eggs 

Young 

Total 

Percentage 
of  young 

Sept    27    

160 

320 

640 

1    120 

57 

."      29  

7,520 

4,000 

12,800 

24,320 

52 

Oct.      5  

3,560 

10,800 

58  400 

72   760 

82 

12 

1   600 

7   600 

9  200 

83 

"      19  

560 

840 

4  440 

5   840 

76 

600  per  m.3are  found  after  the  temperatures  pass  70°,  with  the[single 
exception  of  the  decline  of  the  October  pulse  and  the  rise  of  the 
November  pulse  to  2,040  per  m.3  at  47°,  following  the  high  tempera- 
tures in  the  late  autumn  and  stable  conditions  of  1897.  From  the 
depression  in  numbers  during  the  period  of  maximum  heat  in  mid- 
summer and  the  occurrence  of  the  major  vernal  and  autumnal  pulses 
before  and  after  its  reign  it  appears  that  the  temperature  optimum 
for  D.  cucullata  in  channel  waters  lies  below  this  level,  that  is, 
below  80°. 

D.  cucullata  is  evidently  very  easily  affected  by  the  changes  in 
hydrographic  conditions.  Thus,  in  July-December,  1897  and 
1898,  the  total  movement  in  river  levels  was  12.4  and  31.4  ft., 
respectively,  while  the  total  cucullata  population  for  these  months 
was  186,420  and  1,140 — 164-fold  greater  in  the  more  stable  year. 
D.  cucullata  thus  exhibits  the  maximum  sensitiveness  among  the 
Entomostraca  to  these  environmental  factors. 

The  D.  cucullata  population  in  the  plankton  consists  almost 
entirely  of  parthenogenetic  females  and  young.  The  immature 
stages  form  about  60  per  cent,  and  the  egg-bearing  females  16  per 
cent,  of  the  total  individuals.  Dead,  parasitized,  or  fungused  indi- 
viduals were  found  at  times  of  the  maxima  or  shortly  thereafter, 
but  never  in  very  large  numbers.  Males  were  found  once  in 
December,  1896,  and  ephippial  females  also  but  once,  on  October 
19,  1897,  during  the  decline  of  the  maximum  pulse  in  our  records. 

Daphnia  cucullata  var.  apicata  Kurz,  in  well-developed  condi- 
tion, was  found  in  relatively  small  numbers  during  the  vernal  pulses 
of  1895  and  1896  and  the  autumnal  pulse  of  the  former  year. 


243 

Incipient  stages  of  this  variety  appeared  also  at  other  times.  Burck- 
hardt  ('OOa)  does  not  even  concede  varietal  standing  to  apicata, 
regarding  it  merely  as  a  form  of  seasonal  or  local  value.  Its  occur- 
rence in  our  plankton  when  reproduction  and  growth  are  most 
active  suggests  that  it  may  have  a  growth  value,  and  be  in  some 
way  correlated  with  the  factors  involved  in  its  cyclic  production. 

Daphnia  cucullata  var.  kahlbergiensis  Schoed.  appears  but  once 
in  our  records — in  the  plankton  of  June  11,  1896. 

The  D.  cucullata  group  is  a  cosmopolitan  constituent  of  the 
fresh-wrater  plankton,  appearing  frequently  in  the  records  of  Euro- 
pean plankton.  Apstein  ( '96)  finds  it  in  lakes  in  northern  Germany 
in  April-October  with  maximum  numbers  in  July.  The  seasonal 
limits  thus  resemble  those  in  the  Illinois,  but  the  maximum  falls 
at  the  time  of  our  midsummer  decline.  Temperatures  in  these 
German  lakes  (16.3°  C.)  do  not,  however,  reach  the  high  levels 
attained  in  our  waters  in  midsummer.  Stenroos  ('98)  records  it  in 
several  varieties  in  the  plankton  of  Nurmijarvi  See,  the  helmeted 
varieties  being  found  in  midsummer.  Zacharias  records  it  from 
the  plankton  of  German  ponds.  Scourfield  ('98)  finds  it  in  small 
numbers  in  Epping  Forest  interruptedly  in  April-November,  a 
season  coinciding  with  that  in  the  Illinois.  Burckhardt  ( '00)  finds 
it  represented  by  five  different  "forms"  in  Mauensee  in  the  June 
plankton.  Marsson  ('00)  finds  representatives  of  Hyalodaphnia 
(species  not  given)  in  the  April- June  plankton  near  Berlin.  Am- 
berg  ( '00)  states  that  this  species  appears  in  April,  increasing  to  a 
maximum  in  July- August,  and  disappears  again  at  the  end  of 
November,  a  seasonal  course  similar  in  limits  but  not  in  maximum 
to  that  in  the  Illinois.  His  data  are  too  scattered  to  trace  the  course 
of  production  with  completeness.  Seligo  ('00),  in  waters  near 
Danzig,  finds  the  species  present  in  June- January,  with  maxima 
in  June- July  and  October.  In  the  period  of  maximum  summer 
temperatures  (16°-21°  C.)  the  numbers  decline  as  in  this  period  in 
the  Illinois.  In  Seligo 's  infrequent  (two  to  three  weeks'  interval) 
data  there  are  suggestions  of  minor  recurrent  pulses  in  other  months. 
Cohn  ('03)  finds  in  Lowentin  a  Daphnia  wrhich  he  calls  D.  galeata 
with  vars.  kahlbergiensis  and  cederstromii,  and  includes  all  three  in 
his  enumeration.  His  investigation  covers  the  months  of  May- 
September,  throughout  which  these  forms  appear,  rising  in  a  series 
of  recurrent  maxima  on  June  26,  August  4,  and  September  2  and  29. 

(17) 


244 

Cohn  seems  not  to  have  called  attention  to  these  clearly  denned 
recurrent  pulses. 

In  European  streams  D.  cuciillata  also  forms  an  important  part 
of  the  plankton.  Lauterborn  ('93)  states  that,  with  its  varieties 
kahlbergiensis  and  cederstromii,  it  appears  abundantly  in  the  plank- 
ton of  the  Rhine  in  summer,  but  is  not  found  in  it  in  winter.  Zimmer 
( '99)  states  that  D.  kahlbergiensis  was  found  constantly  in  the  plank- 
ton of  the  Oder  in  July-September,  and  Schorler  ( '00)  also  finds  it 
in  the  Elbe  at  Dresden  in  May- August,  with  larger  numbers  in 
June  and  August.  Steuer  ('01)  reports  it,  in  small  numbers  only, 
in  August  in  the  backwaters  of  the  Danube  at  Vienna.  Fric  and 
Vavra  ('01)  reportD.  kahlbergiensis  as  rare  in  the  Elbe.  Sowinski 
( '88)  finds  it  in  several  varieties  in  plankton  of  the  Dnieper  and  its 
tributaries,  Rossinski  ('92)  finds  it  in  the  summer  plankton  of  the 
Moskwa,  and  Zernow  ('01)  in  the  June-July  plankton  of  the 
Schoschma  and  Wjatka.  Meissner  ('02  and  '03)  finds  it  in  several 
varieties  in  the  May- August  plankton  of  the  Volga. 

D.  cucullata  in  some  of  its  various  forms  or  varieties  appears  to 
be  widely  distributed  in  American  waters.  It  was  reported  by 
Forbes  ('82),  as  D.  retrocurva,  from  the  plankton  of  Lake  Michigan, 
and  also  ('90)  from  Lake  Superior  and  adjacent  waters.  Birge 
('91  and  '94)  also  finds  it  abundantly  in  Wisconsin  waters  and 
in  Lake  St.  Clair.  Herrick  ('84)  and  Ross  ('97)  report  it  from  Min- 
nesota and  Iowa.  Careful  studies  of  its  seasonal  and  vertical 
distribution  in  Wisconsin  waters  have  been  made  by  Marsh  ('97) 
in  Green  Lake,  and  by  Birge  (95  and  '97)  in  Lake  Mendota.  In 
Green  Lake  D.  kahlbergiensis  is  reduced  to  a  minimum  or  even 
extinction  in  December- April,  rises  in  a  late  vernal  maximum  in 
June- July,  falls  again  to  a  lower  level  in  August-September,  and 
then  rises  to  a  second  and  sometimes  higher  autumnal  pulse  in 
October.  In  its  main  outlines  this  conforms  to  the  seasonal  course 
of  the  cucullata  form  in  our  channel  plankton.  Our  vernal  maxi- 
mum appears  somewhat  earlier,  as  a  result  probably  of  an  earlier 
warming  up  of  the  water.  According  to  Birge  ('97)  this  species  is 
more  definitely  periodic  in  its  occurrence  in  Lake  Mendota,  being 
confined  entirely  to  July-December.  Here  also  the  largest  numbers 
are  found  in  October,  and  the  individuals  gather  in  lower  levels  as 
temperatures  decline. 


245 

Daphnia  hyalina  Leydig. — Average  number,  417.  In  channel 
waters  this  species  has  appeared  in  but  two  years,  in  1895  in 
April-July,  attaining  on  June  19  a  maximum  of  166,208  per  m.3,  of 
which  150,626  were  immature.  The  collections  were  too  infrequent 
in  these  months  to  trace  the  course  of  this  vernal  pulse.  D.  hyalina 
did  not  reappear  until  the  spring  of  1898,  on  May  24,  in  a  single 
vernal  pulse  culminating  at  1 1,600  per  m.3  on  June  7,  and  disappear- 
ing a  fortnight  later.  Its  occurrences  with  one  exception  were  all 
at  temperatures  above  70°.  There  is  no  apparent  -  cause  for  its 
absence  in  later  months  or  in  other  years.  Males  and  ephippial 
eggs  were  not  found. 

Daphnia  hyalina  is  an  exceedingly  variable  species,  and  a  large 
number  of  forms  have  been  described  which  belong  to  the  hyaUna 
group.  Burckhardt  ('00),  for  example,  recognizes  26  such  forms 
as  varieties  of  this  cosmopolitan  planktont.  This  variability  and 
the  difficulties  attending  the  resulting  synonymy  cause  any  discus- 
sion of  the  species  in  other  waters  to  be  attended  by  much  uncer- 
tainty. I  shall  therefore  not  attempt  to  distinguish  in  my  dis- 
cussion between  the  various  varieties  included  by  Burckhardt  in 
the  hyalina  group. 

In  lakes  of  northern  Germany,  Apstein  ( '96)  finds  that  D.  hyalina 
is  essentially  a  winter  planktont  with  a  seasonal  range  of  September- 
July,  and  with  maximum  numbers  in  November- January.  The 
maximum  thus  appears  there  at  the  time  of  complete  extinction  in 
our  waters.  Stenroos  ('97)  records  it  (as  D.  galeata)  in  the  summer 
plankton  of  Karelia,  Huitfeldt-Kaas  ('98)  finds  it  in  Norwegian 
lakes  in  July  and  September  in  considerable  numbers,  and  Scour- 
field  's  careful  studies  ( '98)  of  its  seasonal  occurrence  in  waters  of 
Epping  Forest  reveal  an  interrupted  distribution  in  April-Novem- 
ber. vScott  ( '99)  finds  it  in  numbers  in  Scottish  lochs  in  the  plankton 
examined  at  long  intervals  in  March-January.  Fuhrmann  ('00) 
reports  it  as  perennial  in  Neuenbergersee,  with  a  maximum  in  June 
followed  by  a  midsummer  minimum.  Burckhardt  ('OOa)  finds 
great  diversity  in  different  Swiss  lakes  and  in  different  years  in  the 
relative  numbers  present.  His  intervals  of  collection  were  too  great 
to  detect  any  pulse-like  movement  in  the  production,  and  it  may  be 
that  the  diversity  is  due  in  part  to  the  incompleteness  of  his  records. 
He  concludes  that  D.  hyalina  is  at  a  minimum  in  March-May, 
increases  in  numbers  slowly  (with  a  preponderance  of  young  indi- 


246 

viduals)  in  May-October  to  a  maximum  in  November-January, 
which  is  followed  by  a  rapid  decline  (with  preponderance  of  adults) 
to  the  minimum.  His  results  agree  with  those  of  Apstein  ( '96)  in 
the  main  rather  than  with  ours  in  the  Illinois.  Seligo  ('00)  finds 
D.  hyalina  in  Hintersee,  though  it  is  apparently  absent  from  the 
adjacent  Barlewitzersee.  In  the  former  lake  it  appears  in  May, 
rising  to  the  year's  maximum  early  in  June,  continuing  throughout 
the  summer  in  diminished  numbers,  and  disappearing  in  October. 
In  his  infrequent  records  there  are  suggestions  of  several  recurrent 
minor  pulses  during  the  summer.  Cohn  ('03)  reports  D.  galeata— 
regarded  by  Burckhardt  ( 'OOa)  as  a  form  of  D.  hyalina — from  the 
region  of  Konigsberg,  but  refers  it  rather  to  the  cucullata  group.  I 
shall  therefore  consider  his  results  only  in  connection  with  D. 
cucullata. 

D.  hyalina  appears  but  rarely  in  the  records  of  European  potamo- 
plankton.  Steuer  ('01)  reports  it,  in  small  numbers  only,  in  May 
from  the 'backwaters  of  the  Danube  at  Vienna.  Fric  and  Vavra 
('01)  state  that  D.  microcephala — regarded  by  Burckhardt  ('OOa) 
as  a  form  of  D.  hyalina — is  abundant  in  the  plankton  at  a  depth  of 
0-1  m.  in  April-November  in  the  Elbe  and  its  backwaters  at 
Podiebrad.  It  is  also  reported  by  Zykoff  ('00  and  '03)  in  the  late 
vernal  (June- July)  plankton  of  the  Volga  at  Saratoff,  and  by  Meissner 
('02  and  '03)  in  the  same  stream  in  May- June.  The  examination 
of  the  plankton  of  the  Volga  made  by  these  authors  is  far  less 
extensive  than  that  made  of  the  Illinois  River  plankton,  but  as  far 
as  it  goes  it  indicates  a  similar  distribution  of  D.  hyalina  in  the  two 
streams.  Volk  ('03)  reports  it  from  the  Elbe  at  Hamburg  without 
data. 

The  species  appears  to  be  widely  distributed  in  American  waters, 
being  reported,  in  some  of  its  various  varieties  or  synonyms,  especially 
from  lakes  and  ponds.  Smith  ( '74)  finds  it  in  the  plankton  of  Lake 
Superior,  Forbes  ('82)  in  that  of  Lake  Michigan,  and  Birge  ('94) 
in  Lake  St.  Clair.  It  was  also  found  in  the  Illinois  by  Forbes  ('78) 
and  in  the  backwaters  of  the  Ohio  River  by  Herrick  ('84),  who 
reports  it  also  from  Minnesota  waters.  Birge  ('91)  finds  it  in  lakes 
about  Madison,  Wis.,  and  Fordyce  ('00)  in  deep  pools  in  western 
Nebraska.  The  only  investigation  of  its  seasonal  distribution  in 
American  waters  is  that  of  Birge  ('95  and '97)  in  Lake  Mendota, 
where  it  forms  about  3  per  cent,  of  all  the  Crustacea.  It  is  perennial 


247 

in  this  lake  but  exhibits  great  differences  in  its  seasonal  course  from 
year  to  year.  The  vernal  development  in  May-June  (the  only  one 
in  our  channel  plankton)  is  relatively  large  in  each  year,  but  is 
sometimes  exceeded  by  an  autumnal  one  in  October.  A  midsummer 
minimum  sometimes  appears  between  these  pulses,  and  a  winter 
minimum  in  December- April  is  always  present. 

From  the  data  here  reviewed  it  seems  probable  that  the  very 
limited  seasonal  distribution  and  irregular  annual  recurrence  of 
D.  hyalina  in  our  channel  plankton  is  in  a  measure  indicated  in 
streams  elsewhere,  and  may  have  its  cause  in  the  instability  of  the 
fluviatile  environment  as  compared  with  the  lacustrine,  where  the 
species  evidently  finds  its  environmental  optimum. 

Diaphanosoma  brachyurum  (Lievin). — Average  number,  479,  of 
which  154  are  females,  49  females  with  eggs,  and  276  immature. 

This  species  in  our  waters  is  monocyclic,  with  sharply  defined 
seasonal  distribution.  With  the  exception  of  two  records  of  young 
individuals  in  March- April,  1895  (and  the  identification  of  these 
individuals  is  questionable),  all  our  records  of  occurrence  in 
1894-1899  fall  between  May  25  and  October  19,  the  first  vernal 
records  appearing  at  temperatures  of  55.8°  to  72.3°,  and  the  last 
autumnal  at  52.5°  to  65°.  The  one  pulse  in  each  year — except  in 
1894,  when  none  was  recorded — falls  in  a  period  of  3-6  weeks  in 
July-September,  the  first  record  above  2,000  per  m.3  appearing 
July  26,  and  the  latest  (with  one  exception,  2,175  on  September  27, 
1895)  on  September  7.  The  pulse  varies  in  duration  in  different 
years  from  3  to  6  weeks,  and  attains  a  maximum  on  dates  ranging 
from  July  26  to  August  31,  and  varying  in  amplitude  from  8,580 
to  19,602  per  m.3  An  analysis  of  the  distribution  of  61  recorded 
occurrences  in  channel  plankton  shows  that  of  these  only  13,  or  21 
per  cent.,  occur  outside  of  July-September,  and  that  the  records 
outside  of  the  seven  weeks  of  the  pulse  include  less  than  12  per  cent, 
of  the  total  individuals. 

A  comparison  of  the  seasonal  curve  of  distribution  with  the 
annual  thermograph  reveals  the  fact  that  the  pulse  occurs  toward 
the  close  of  the  period  of  maximum  summer  heat,  and  in  every  case 
at  a  temperature  of  78°  or  above,  and  that  the  decline  of  the  pulse 
often  begins  with  declining  temperatures,  and  is  always  accom- 
plished during  the  autumnal  decline.  The  effect  of  summer  heat 
pulses  upon  the  Diaphanosoma  curve  is  strongly  suggested  by  the 


248 


data  of  the  appended  table,  which  gives  the  statistics  of  temperature, 
river  level,  and  Diaphanosoma  population  during  the  periods  of 
maximum  development  in  1895-1898.  All  these  data  except  those 
of  Diaphanosoma  are  shown  graphically  in  Part  I.,  Plates  IX.-XII. 
The  data  for  Diaphanosoma  are  less  complete  than  the  others,  since 
all  of  the  collections  were  not  counted. 

In  1895  the  Diaphanosoma  pulse  culminates  at  19,602  on  August 
21,  following  immediately  upon  a  heat  pulse  which  culminates 
August  15  at  85.3°.  The  decline  of  the  pulse  occurs  with  a  decline 
of  temperature  to  72°  on  September  7.  The  declines,  both  of 
Diaphanosoma  and  temperature,  are  hastened  after  September  3  by 


1895 

1896 

Date 

River 
gage 

Temp. 

No.  of 
Diaph- 
anosoma 

Date 

River 
gage 

Temp. 

No.  of 
Diaph- 
anosoma 

July     2 
"       1  n 

5.15 
4nn 

80.8 
7r>    e 

40 
cnn 

"           1  Q 

2     en              ic\ 

July   23 

5.20 

80 

424 

"       23 

4.20 

80 

120 

"      29 

5.38 

75.5 

240 

"       28 

6.40 

82 

7,440 

Aug.     1 

4.20 

78.5 

1,088 

Aug.     3 

8.50 

80.3 

160 

8 

2.63 

79 

988 

8 

8.40 

86 

14,260 

"       12 

2.40 

84.8 

9,801 

"       15 

7.40 

82 

2,240 

"       21 

2.08 

81.5 

19,662 

"       21 

7.10 

79 

880 

"       29 

2.58 

80 

7,950 

"       26 

'•            Of) 

6.50 

77.5 

7/1      1 

600 

Sept.    5 

5.70 

74 

189 

Sept.  16 

4.10 

73.5 

663 

"      12 

3.90 

79 

1,053 

"      30 

4.30 

58 

80 

"       20 

7  ?n 

7Q 

468 

"      27 

3.23 

73 

2,175 

1 

249 


1897 

1898 

Date 

River 
gage 

Temp. 

No.  of 
Diaph- 
anosoma 

Date 

River 
gage 

Temp. 

No.  of 
Diaph- 
anosoma 

July    14 

6.30 

79 

160 

July    12 

7.00 

78 

60 

"       21 

5.20 

81.1 

960 

"       19 

4.70 

84 

40 

"       30 

4.60 

84 

4,720 

"       26 

2.90 

89 

8,580 

Aug.   10 

2.30 

80.8 

7,600 

Aug.     2 

2.70 

78.3 

6,960 

"       17 

1.90 

79 

7,120 

9 

3.20 

83 

360 

"       24 

1.80 

77.5 

5,120 

"       16 

3.70 

77 

60 

"       31 

1.80 

80 

1  1  ,  000 

"      23 

4.20 

82 

1,020 

»*         3O 

7    on 

09    cr 

7     con 

Sept.     7 

1.80 

80 

7,600 

Sept.    6 

4.70 

79 

240 

"      14 

2.00 

83 

1,500 

"      13 

4.20 

62.5 

1,800 

"      21 

2.00 

71 

240 

"      20 

4.20 

73 

960 

"      27 

4.90 

73 

400 

the  rise  in  river  levels.  Prior  to  that  date  hydrographic  changes 
are  slight.  With  falling  levels  and  higher  temperatures  after 
September  7  there  is  a  slight  recovery  in  Diaphanosoma — from  189 
per  m.3  on  the  5th,  to  1,053  on  the  12th. 

In  1896  a  well-defined  heat  pulse  culminates  August  10  at  86.5°; 
and  Diaphanosoma,  on  August  8  at  14,260,  with  an  abrupt  depression 
from  7,440,  on  July  28,  to  160,  on  August  3,  in  flood  waters.  The 
decline  of  this  pulse  from  the  maximum  on  the  8th  to  440  on  the  29th 
is  attended  by  a  uniform  decline  in  temperatures  from  86°  to  74.3°  in 
fairly  stable  hydrographic  conditions,  that  is,  declining  river  levels. 

In  1897  there  are  two  well-defined  summer  heat  pulses,  one 
culminating  August  3  at  89°,  and  the  other  September  14  at  83°, 
separated  by  a  depression  to  77.5°  on  August  24.  The  crest  of  the 
Diaphanosoma  pulse  likewise  has  two  apices,  the  first  culminating 
at  7,600  on  August  10,  followed,  during  the  decline  in  temperatures, 


250 

by  a  fall  to  5,120  on  the  24th,  and,  in  the  rising  temperatures  which 
then  ensue,  by  a  recovery  to  a  second  maximum  of  11,000  on  the 
31st.  Diaphanosoma  then  declines  though  temperatures  continue 
to  rise.  These  fluctuations  all  take  place  in  comparatively  stable 
hydrographic  conditions.  There  is  a  suggestion  in  the  records  of 
this  year  that  rising  temperatures  in  midsummer  conditions  tend 
to  accelerate,  and  falling  temperatures  to  depress,  development  of 
the  Diaphanosoma  pulse,  and  also  that  after  the  pulse  has  continued 
for  some  time  (six  weeks  in  this  instance)  rise  in  temperature  ceases 
to  be  effective.  The  autumnal  decline  in  Diaphanosoma  may 
therefore  not  always  of  necessity  be  due  to  temperature  decline  alone. 

In  1898  there  are  also  two  midsummer  heat  pulses,  culminating 
on  July  26  at  89°,  and  August  30  at  82.5°,  separated  by  a  depression 
which  reaches  77°  on  August  16.  The  depression  to  78.3°  on  August 
2,  with  the  consequent  appearance  of  a  third  summit  at  83°  on 
August  9,  is  due  mainly  to  the  fact  that  the  temperature  was  taken 
at  9:15  a.  m.,  while  all  the  others  were  in  the  late  afternoon.  The 
seasonal  curve  of  Diaphanosoma  shows  likewise  two  apices,  the  first 
at  8,580  on  July  26,  and  the  second  at  2,520  on  August  30,  separated 
by  a  depression  to  60  per  m.3  on  August  16,  when  temperatures  are 
lowest.  In  this  year  the  flood  of  the  middle  of  August  doubtless 
plays  a  large  part  in  depressing  alike  the  thermograph  and  the 
seasonal  curve  of  Diaphanosoma,  but  in  the  light  of  the  evidence 
from  1897  in  stable  hydrographic  conditions  the  direct  influence 
of  temperature  is  also  possible  in  this  instance. 

Diaphanosoma  is  thus  a  late  summer  planktont  wrhich  in  develop- 
ment is  very  responsive  to  changes  in  temperature.  It  appears  in 
the  plankton  in  small  numbers  shortly  after  the  establishment  of 
summer  temperatures  in  May- June,  but  does  not  begin  its  maximum 
development  until  maximum  summer  temperatures  have  existed 
for  six  to  eight  weeks,  and  is  apparently  incited  to  this  by  a  summer 
heat  pulse. 

Males  were  recorded  on  July  18  and  August  1,  and  ephippial 
females  on  August  1  and  September  5.  Dead  individuals  were 
most  numerous  during  or  subsequent  to  the  maximum  of  the  pulse. 

This  species  is  reported  by  Apstein  ('96)  in  the  plankton  of 
Dobersdorfersee,  where  it  is  also  monocyclic,  first  appearing  in 
May,  and  attaining  its  maximum  in  September,  when  the  males 
first  appear.  In  contrast  with  conditions  in  our  waters  the  maxima 


251 

appear  after  the  period  of  maximum  summer  heat.  Zacharias  ('97a) 
reports  it  from  German  carp  ponds  in  July,  and  Stenroos  ('97)  lists 
it  as  a  littoral  species  in  midsummer  in  northern  Russia.  Scott  ( '99) 
finds  it  rarely  in  lakes  of  Scotland  in  August,  and  then  only  in  the 
plankton,  though  many  shore  collections  were  examined.  Burck- 
hardt  ( '00)  reports  it  from  the  smaller  and  shallower  Swiss  lakes  in 
isolated  records  ranging  from  May  to  November,  and  regards  its 
absence  from  the  deeper  lakes  as  due  to  the  low  temperatures  which 
at  all  seasons  would  surround  its  winter  eggs,  which  sink  to  the  lowrer 
levels.  In  Vierwaldstattersee  ('OOa)  he  finds  this  species  in  the 
plankton  only  in  September-November,  and  then  more  abundantly 
near  shore  than  in  the  middle  of  the  lake.  In  Alpnachersee  the 
period  of  occurrence  extends  from  June  to  November  with  a  maxi- 
mum in  July.  Fuhrmann  ('00)  gives  the  seasonal  distribution  in 
Neuenburgersee  as  extending  from  May  to  November,  with  a  maxi- 
mum in  September.  Marsson  ('00)  finds  a  seasonal  distribution 
from  July  to  October  in  small  lakes  near  Berlin.  Seligo  ( '00)  finds 
in  Hintersee,  near  Danzig,  a  seasonal  distribution  in  1898  extending 
from  June  6  to  October  18,  with  a  maximum  of  225,000 — under 
1  sq.  m.,  depth,  24  m.  (?) — on  August  9.  Fric  and  Vavra  ( '01 )  state 
that  this  species  is  very  abundant  in  summer  months  in  the  plank- 
ton of  the  backwaters  of  the  Elbe,  especially  in  levels  at  depths 
of  0-1  meter.  Cohn  ('03),  on  the  other  hand,  finds  in  waters  near 
Konigsberg  that  Diaphanosoma  is  present  in  greatest  abundance 
in  depths  of  20-30  meters.  It  occurs  in  summer  months,  with  large 
numbers  in  July-September  and  a  maximum  in  August-September. 
It  was  not  found  in  shallow  waters. 

As  a  constituent  of  the  potamoplankton  Diaphanosoma  has  been 
reported  by  Schorler  ('00)  in  the  Elbe  at  Dresden  as  abundant  in 
June-September.  Steuer  ('01)  finds  it  in  the  backwaters  of  the 
Danube  at  Vienna  in  June-September,  with  a  maximum  in  August, 
but  never  in  great  numbers.  Meissner  ('03)  reports  it  sparingly 
from  the  Volga  in  July. 

In  American  waters  Diaphanosoma  is  widely  distributed.  Forbes 
('90)  found  it  abundant  below  surface  levels  in  Lake  Michigamme 
in  August.  Birge  ( '94)  reports  it  in  the  plankton  of  western  Lake 
Erie  but  not  in  that  of  Lake  St.  Clair  in  September.  In  Lake 
Mendota,  Wis.,  he  ('95  and  '97)  has  worked  out  its  seasonal  and 
vertical  distribution  with  a  fulness  and  care  not  equaled  by  any 


252 

European  author  previously  quoted.  Our  results  in  Illinois  waters 
are  in  striking  confirmation  of  his  conclusions.  He  finds  the  first 
scattering  individuals  in  the  plankton  late  in  May,  but  numbers  do 
not  rise  until  late  in  July  or  early  in  August,  increasing  rapidly 
through  August  or  even  into  September,  then  declining  rapidly,  and 
disappearing  entirely  before  November  1 .  The  active  period  is  thus 
at  a  time  when  a  considerable  part  of  the  lake  is  at  or  above  68°.  In 
our  waters  these  temperature  limits  are  78°  or  above,  but  the  sea- 
sonal distribution  is  almost  identical  with  that  in  Lake  Mendota.  He 
finds  it  more  abundant  in  the  upper  strata,  0-2  meters,  than  in  the 
deeper  ones — just  the  opposite  of  Cohn's  ('03)  results.  Marsh 
('97)  has  also  determined  its  seasonal  and  vertical  distribution  in 
Green  Lake,  Wis.,  with  considerable  care.  Occurrences  from  the 
last  of  October  to  the  last  of  June  are  very  few,  and  maximum 
numbers  appear  from  the  middle  of  August  to  the  middle  of  Septem- 
ber, when  surface  waters  have  a  temperature  of  65°-80°.  It  occurs 
in  all  depths  (0-40  m.),  but  70  to  80  per  cent,  of  the  individuals 
were  taken  within  10  to  15  m.of  the  surface,  the  upper  5  meters  being 
more  densely  populated  by  night  than. by  day  and  in  September- 
October  than  in  August. 

Diaphanosoma  is  a  typical  planktont,  with  strong  antennae,  and 
an  active  swimmer.  Examination  of  the  literature  indicates  its 
wide  distribution  in  the  plankton  of  lakes  and  streams,  and  its  very 
marked  seasonal  limitation  to  seasons  of  higher  temperature.  It  is 
thus,  as  Birge  ('97)  has  stated,  markedly  stenothermous.  The 
divergent  conclusions  concerning  its  limnetic  habit  and  its  vertical 
distribution  will  doubtless  be  found  to  rest  in  some  cases  upon 
insufficient  data,  and  in  others,  upon  its  reactions  to  varying  condi- 
tions of  light  and  temperature. 

Eurycercus  lamellatus  O.  F.  Mull. — This  species  occurred  spar- 
ingly and  irregularly  in  the  winter  plankton  at  minimum  tempera- 
tures from  November  30  to  March  28.  It  is  evidently  adventitious. 

Ilyocryptus  spinifer  Herrick. — Average  number,  4.  This  species 
occurred  sparingly  and  irregularly  in  the  plankton  during  the 
warmer  months.  The  earliest  record  was  on  July  23,  and  the  latest 
October  11  at  65°.  This  species  is  evidently  adventitious  in  the 
plankton.  I  have  doubtfully  referred  our  examples  to  Herrick 's 
species  /.  spinifer,  for  the  reasons  given  by  Herrick  and  Turner  ('95), 
rather  than  to  /.  longiremis,  to  which  Birge  ('91)  would  refer  our 


253 

American  form  described  by  Herrick  as  /.  spinifer.  A  larger  amount 
of  material  exhibiting  a  fuller  range  of  variation  may,  however,  serve 
to  connect  the  two. 

Leptodora  hyalina  Lilljeborg. — Average  number,  3.  This  species 
occurred  in  small  numbers  and  somewhat  irregularly  in  our  collec- 
tions of  channel  plankton  in  summer  months.  Our  earliest  record 
was  June  28 ;  and  the  latest,  August  30.  It  is  our  largest  crustacean 
planktont  and  a  fairly  active  swimmer,  and  was  often  taken  in  our 
tow-nets,  which  had  a  larger  mouth  and  coarser  mesh  (No.  12)  than 
our  plankton  net.  I  took  this  species  in  great  numbers  in  the  upper 
meter  of  water  at  midday  in  May- June  in  Lake  Meredosia  with  a 
seine  of  No.  000  silk.  It  may  be  that  it  is  less  abundant  in  the 
channel  than  in  the  backwaters,  and  the  small  number  in  the  plank- 
ton collections  from  the  channel  may  also  be  accounted  for  in  part 
by  the  escape  of  Leptodora  from  the  small  orifice  (10  cm.)  of  the 
plankton  net,  or  to  its  negative  rheotropism  when  stimulated  by  the 
currents  of  the  plankton  pump. 

Macroihrix  laticornis  Jurine  was  found  in  the  plankton  in  May 
at  64°-73°,  adventitious  in  flood  waters. 

Moina  micrura  Kurz. — Average  number,  261  per  m.3  In  1897 
it  was  much  more  abundant,  averaging  5,106  in  the  more  stable 
conditions  of  that  year. 

This  is  the  most  abundant  of  all  our  Cladocera,  appearing  in 
great  numbers  in  periods  of  stable  low  water  during  maximum 
temperatures.  It  is  exceedingly  irregular  in  the  extent  of  its  devel- 
opment in  different  years,  the  average  numbers  per  m.3  in  1894-1898 
being  respectively  21,844,  22,842,  188,  5,106,  and  261.  After  mak- 
ing allowances  for  the  irregularity  in  the  number  and  distribution 
of  the  collections  in  the  several  years,  it  still  remains  apparent  that 
Moina  is  very  uneven  in  its  distribution. 

The  seasonal  distribution  of  Moina  in  channel  plankton  is  con- 
fined to  July-September  with  the  exception  of  9  occurrences  in 
small  mmibers  in  the  last  days  of  June  and  the  early  part  of  October. 
The  earliest  record  is  June  19,  in  1895,  wrhen  the  very  large  number 
of  329,448  per  m.3  were  found, — a  degree  of  development  which 
implies  a  previous  period  of  multiplication.  The  first  records  in 
subsequent  years  were  all  later  than  this  date  in  June  or  early  in 
July.  After  several  recurrent  pulses,  each  of  3  to  5  weeks '  duration, 
the  numbers  decline  to  a  very  low  level,  and  the  species  disappears 


254 


from  the  plankton  in  September-October.  In  1898  (Table  I.)  the 
last  record  was  made  October  11 — the  latest  in  any  year  with  the 
exception  of  an  isolated  record  October  26,  1897.  Moina  micrura 
is  thus  distinctly  a  summer  planktont. 

It  appears  in  the  plankton  only  after  maximum  summer  tem- 
peratures of  approximately  80°  have  been  reached,  and  decreases 
rapidly  as  soon  as  the  autumnal  decline  passes  this  point,  and  soon 
thereafter  vanishes  from  the  plankton.  Its  optimum  temperature 
in  channel  waters  is  thus  near  80°. 

The  relation  which  hydrographic  conditions  bear  to  the  ap- 
pearance of  Moina  in  channel  plankton  appears  upon  a  comparison 
of  the  Moina  population  and  the  movement  in  river  levels  in  differ- 
ent years,  as  shown  in  the  following  table. 

MOINA  AND  HYDROGRAPHIC  CHANGES. 


June 

July 

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September 

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1894 

192 

3.4 

40,415 

2.1 

129,880 

2.6 

3,677 

4.7 

0 

3.1 

1895 

329,448 

2.7 

91,318 

7.3 

2,597 

3.5 

87 

8.8 

10 

2.7 

1896 

0 

3.4 

152 

7.8 

1,220 

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1897 

0 

6.3 

1,373 

5.2 

1,280 

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1898 

75 

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660 

7.4 

1,496 

7.5 

770 

6.2 

40 

3.9 

While  the  correlation  is  not  proportionate  between  the  extent  of 
movement  in  levels  and  the  Moina  per  m.3,  it  is  still  very  evident 
that  in  years  of  continued  and  more  stable  low  water  Moina  is 
found  in  much  greater  numbers,  as  appears  on  a  comparison  of  1897 
and  1898.  It  is  also  confined  largely  to  the  more  stable  part  of  the 
year,  appearing  in  1895  in  June- July  in  large  numbers,  but  falling 
off  when  the  minor  floods  of  August-September  occur,  while  in  1897 
the  large  numbers  are  found  in  the  stable  levels  of  August. 


255 

The  cause  of  this  limitation  of  Moina  to  periods  of  low  levels  in 
maximum  temperatures  appears  to  lie  in  the  food  relations  of  the 
species.  Moina  abounds  in  waters  approaching  stagnation.  The 
slackened  current,  increased  sewage  contamination,  and  excessive 
growth  of  the  smaller  algas  and  chlorophyll-bearing  flagellates  at 
such  seasons  in  the  channel  of  the  Illinois  furnish  an  environment 
favorable  to  the  great  increase  in  Moina,  such  as  was  recorded  in 
the  low  water  of  July- August,  1894,  of  June- July,  1895,  and  of 
September,  1897,  exceeding  in  each  instance  that  of  any  other 
species  of  Entomostraca  in  the  plankton.  The  relatively  smaller 
numbers  of  Moina  at  the  same  seasons  in  the  less  contaminated 
backwaters  lends  additional  support  to  the  view  that  these  condi- 
tions approaching  stagnation  are  in  a  measure  responsible  for  its 
unusual  development  in  channel  plankton. 

Of  the  total  Moina  population,  over  65  per  cent,  are  young  or 
immature,  7  per  cent,  are  egg-bearing  females, — embryos  are  often 
freed  from  the  parent  on  application  of  the  preserving  fluid, — 11  per 
cent,  are  males,  and  the  remainder,  females  without  eggs.  Males 
appeared  with  the  maximum  or  decline  of  the  major  pulse  for  the 
year  in  1894  (August),  1895  (July),  1897  (July  and  September),  and 
1898  (September),  but  ephippial  females  were  recorded  only  in 
June-July,  1895. 

The  seasonal  distribution  of  Moina  conforms  to  the  type  of  a 
series  of  recurrent  pulses  wherever  the  numbers  are  considerable 
and  the  collections  sufficiently  frequent  to  delineate  their  courses. 
Even  in  the  small  numbers  of  1898  (Table  I.)  there  are  suggestions 
of  such  pulses. 

Moina  micrura  seems  to  be  a  species  characteristic  of  the  pota- 
moplankton.  It  is  not  mentioned  as  a  constituent  of  the  plankton 
or  littoral  fauna  by  any  of  the  various  investigators  quoted  else- 
where in  this  paper  who  deal  with  lakes  or  ponds  in  Europe  or  North 
America;  nor  does  it  appear  as  a  frequent  constituent  of  the 
potamoplankton  elsewhere.  Skorikow  ('02),  indeed,  makes  the 
statement,  "  Bemerkenswert  ist  fur  die  Fliisse  vollstandiges  Fehlen 
der  Gattung  Moina."  This,  however,  is  hardly  the  case,  for 
Sowinski  ("88)  finds  it  in  the  plankton  of  the  Teterew,  a  tributary 
of  the  Dnieper,  and  Fric  and  Vavra  ('01)  report  it  from  the  Elbe 
in  0-1  m.  strata  in  July-September,  males  appearing  in  the  latter 
month.  Meissner  ( '02  and  '03)  also  finds  it  in  the  Volga  at  Saratoff, 


256 

where  it  "  appears  almost  constantly  in  the  plankton."  His  investi- 
gations, however,  appear  to  cover  only  the  months  of  May-August. 
Maximum  numbers  appeared  in  July,  and  considerable  differences 
were  noted  in  two  successive  years. 

I  find  no  previous  record  of  the  occurrence  of  Moina  nticrura  in 
American  waters. 

Pleuroxus  denticulatus  Birge. — Average  number,  5.  Occurs  in 
small  numbers  and  irregularly  during  the  autumn  and  spring  months 
during  declining  or  rising  temperatures.  The  earliest  autumnal 
record  is  November  2,  and  the  latest,  December  15;  the  earliest 
vernal  is  March  8,  and  the  latest  is  May  31.  Egg-bearing  females 
appear  in  the  earlier  occurrences  in  each  season.  It  is  evidently 
adventitious. 

Pleuroxus  hamatus  Birge  was  found  once — March  29,  1898. 

Scapholeberis  mucronata  O.  F.  Mull,  was  recorded  in  small  num- 
bers in  May  and  August-December  through  the  seasonal  range  of 
temperatures.  It  is  apparently  adventitious  in  channel  plankton, 
though  not  attending  flood  invasions. 

Sida  crystallina  O.  F.  Mull,  is  rare  in  the  summer  plankton. 

Simocephalus  serrulatus  Koch. — Average  number,  261.  This 
species  appears  irregularly  in  the  plankton,  generally  in  small 
numbers  and  in  isolated  occurrences.  An  exception  to  this  is  found 
in  May- June,  1898  (Table  I.),  when  it  is  found  continuously  May 
10- June  14  in  numbers  which  furnish  61  per  cent,  of  the  total  for 
all  years.  There  is  a  slight  preponderance  of  occurrences  in  May 
and  September,  12  of  the  26  recorded  appearing  in  these  months. 
Their  irregular  appearance  in  the  plankton  in  general  suggests  that 
they  are  adventitious  from  the  littoral  area,  especially  at  times 
of  their  maximum  development  there.  The  period  of  their  occur- 
rence in  the  channel  plankton  in  1898  was  one  of  rising  water,  10  to 
14  feet  above  low-water  mark — a  stage  permitting  free  communica- 
tion between  the  channel  and  large  areas  of  slightly  submerged 
bottom-lands. 

Simocephalus  vetulus  O.  F.  Mull,  appeared  irregularly  and  in 
small  numbers  in  the  plankton  in  April- June  (4  occurrences)  and 
September-December  (5  occurrences).  It  is  evidently  adventitious 
in  the  plankton,  coming  from  the  littoral  area,  though  not  confined 
to  flood  waters. 


257 


i  OSTRACODA. 

The  species  of  this  order  are  in  the  main,  during  adult  life, 
limicolous  forms  found  in  the  littoral  or  bottom  ooze  or  amid  the 
decaying  organic  matter  which  accumulates  in  these  regions.  The 
current,  the  movements  of  fish  and  other  large  aquatic  organisms, 
the  action  of  waves  along  shore  and  in  shoal  regions,  all  tend  to  bring 
these  animals  into  the  limnetic  fauna.  Their  centers  of  distribution 
are  thus  in  littoral  or  bottom  regions,  and  in  the  adult  stage  they 
are  almost  wholly  adventitious  in  the  plankton  of  our  waters.  In 
1898  the  average  number  per  m.3  was  191,  but  in  1897,  a  more  stable 
year,  only  97. 

The  seasonal  distribution  of  their  occurrences  in  the  plankton 
indicates  a  decided  predominance  in  March-October,  in  which 
months  all  but  6  of  the  73  records  were  made.  In  these  months 
from  23  to  82  per  cent,  of  the  collections  contained  Ostracoda,  while 
in  December-February  only  8  to  20  per  cent.  The  percentages  in 
April-September  are  all  above  45  per  cent.,  and  the  numbers  per 
m.3  are  also  larger  in  this  period  (see  Table  I.).  The  tendency 
toward  a  vernal  increase  is  apparent  in  the  records  of  each  year  in 
much  the  form  in  which  it  occurs  in  1898  (Table  I.).  The  numbers 
are  always  small  at  all  seasons,  not  exceeding  1,600  per  m.3  even 
in  the  vernal  season. 

The  seasonal  distribution  is  such  that  the  greater  part  of  the 
occurrences  and  the  greater  number  of  individuals  appear  in  the 
plankton  during  the  warm  season,  that  is,  above  50°.  Thus,  in  1898 
all  but  4  of  the  24  occurrences  and  99.5  per  cent,  of  the  indivi- 
duals appear  after  the  vernal  rise  passes  50°  and  before  the 
autumnal  decline  reaches  that  point.  The  Ostracoda  are  plank- 
tonts  of  the  warmer  season. 

It  is  significant  that  the  Ostracoda  in  our  plankton  collections 
are  largely  young  or  immature  individuals.  In  1898, 'for  example, 
74  per  cent,  of  individuals  observed  were  not  adult,  and  most  of 
these  appeared  in  April-June.  Their  occurrence  in  the  plankton 
can  not  be  traced  to  the  action  of  flood  waters.  It  thus  seems 
probable  that  the  young  Ostracoda  may  temporarily  adopt  more 
of  a  limnetic  habit  than  the  adults. 

No  attempt  was  made  to  systematically  identify  the  Ostracoda  of 
the  plankton  catches.  The  list  of  species  and  the  notes  thereon 


258 

which  follow,  are  drawn  in  the  main  from  Sharpe  ('97),  to  whom  I 
am  also  indebted  for  assistance  in  identifications  which  I  have 
made.  A  few  supplementary  notes  are  based  on  my  plankton 
.records. 

DISCUSSION    OF    SPECIES    OF    OSTRACODA. 

Candona  sigmoides  Sharpe.  is  rare  in  shore  collections  below  the 
plankton  station. 

Candona  reftexa  Sharpe  was  taken  but  once  in  the  river — on 
November  11. 

Candona  simpsoni  Sharpe  appears  commonly  in  April-May,  and 
again,  in  smaller  number,  in  October-November  in  shore  collections 
on  the  west  side  of  the  river  at  the  plankton  station.  It  is  occa- 
sionally adventitious  in  the  plankton  at  these  seasons. 

Cypria  exsculpta  Fischer  appears  rarely  in  the  channel  plankton 
and  in  shore  collections  in  April-October. 

Cypria  ophthalmica  Jurine  is  found  frequently  in  the  plankton 
throughout  the  year,  but  more  abundantly  in  May-September,  and 
especially  in  late  summer  and  early  autumn. 

Cypria  pustulosa  Sharpe  was  taken  rarely  in  channel  plankton 
in  July  and  September. 

Cypridopsis  vidua  O.  F.  Mull,  was  perennial  in  the  plankton, 
though  present  in  greater  numbers  in  May-October.  It  is  the 
commonest  of  the  Ostracoda  in  the  plankton,  and  it  seems  probable 
that  many,  though  not  all,  of  the  young  and  immature  forms  belong 
to  this  species. 

Limnicythere  illinoisensis  Sharpe  was  taken  in  the  plankton  in 
March,  August,  and  November  in  1898,  in  two  instances  in  flood 
waters. 

COPEPODA. 

This  is  the  most  abundantly  represented  order  of  the  Entomos- 
traca  in  channel  plankton.  Though  the  species  number  but  12  to 
the  25  Cladocera,  the  individuals  among  the  Copepoda  outnumber 
the  Cladocera  over  fivefold  in  the  grand  totals,  the  ratio  varying  in 
individual  years  from  twofold  in  1894  to  almost  sevenfold  in  1898. 

The  average  number  in  1898  was  40,608  per  m.3;  in  1897,  in 
more  stable  conditions,  80,632;  in  1896,  a  year  of  recurrent  floods, 
43,764 — approximately  the  number  in  1898;  in  1895,  a  year  of  low 
wrater  in  spring,  116,264 — the  highest  average  of  any  year;  and  in 


259 

1894,  53,149.  On  June  19,  1895,  the  Copepoda  attained  a  vernal 
maximum  of  1,022,476  per  m.3 — more  than  twice  the  maximum 
record  for  any  other  year. 

The  Copepoda  occur  in  every  collection  examined,  and  throughout 
the  whole  seasonal  range  in  temperatures.  As  shown  in  Table  I., 
the  copepodan  population  during  minimum  temperatures  in  De- 
cember-February is  at  a  minimum,  the  number  per  m.3  rising  above 
10,000  per  m.3  in  but  6  instances  in  44  collections  in  these  months, 
and  falling  below  1,000  in  but  5.  In  March- April,  as  temperatures 
rise,  the  numbers  increase  rapidly,  especially  after  50°  is  passed,  to 
a  vernal  maximum  in  the  last  days  of  April  or  early  in  May,  usually 
at  the  time  of  the  vernal  volumetric  maximum  or  very  shortly 
thereafter.  In  fact,  volumetric  maxima  are  generally  accompanied 
by  copepodan  maxima  culminating  at  the  same  time  or  a  week 
later, — as  in  May,  1898,  when  the  volumetric  is  on  May  3  and  the 
copepodan  on  May  10. 

Numbers  continue  to  be  large  during  the  period  of  summer 
heat,  declining  somewhat  tardily  with  the  autumnal  decline  in 
temperatures.  In  midsummer  in  1898  numbers  fall  below  20,000 
in  9  instances  in  disturbed  hydrographic  conditions,  but  in  all 
previous  years  in  April-September  there  are  only  9  such  records  in 
a  total  of  63.  The  decline  to  the  winter  minimum  is  usually  com- 
pleted in  November,  though  in  1897,  20,000  is  not  permanently 
passed  until  December  21,  at  32°. 

The  Copepoda  are  thus  perennial  in  the  plankton,  and  the  fact 
that  they  exhibit  a  larger  winter  population  than  the  Cladocera  is 
due  to  the  fact  that  a  number  of  species, — the  Harpacticida,  Cyclops 
bicuspidatus,  C.  prasinus,  C.  serrulatus,  and  C.  modestus  appear  to 
be  planktonts  belonging  to  the  colder  part  of  the  year.  As  a  whole, 
however,  the  Copepoda  reach  their  greatest  quantitative  develop- 
ment in  the  warmer  part  of  the  year,  with  a  major  pulse  in  April- 
May  and  an  occasional  autumnal  pulse,  as  in  1897,  of  equal  or 
greater  proportions. 

The  whole  course  of  the  seasonal  occurrence  of  the  Copepoda  as 
revealed  by  collections  at  frequent  intervals,  exhibits  the  phenome- 
non of  recurrent  pulses  at  intervals  of  3  to  6  weeks,  and  more  clearly 
denned  in  stable  conditions.  Owing  to  their  relatively  smaller 
numbers  the  adult  Copepoda  do  not  show  the  pulse  phenomenon 


(  18) 


260 


as  clearly  as  the  nauplii  and  immature  forms.     In  1898  the  adults 
form  only  10  per  cent,  of  the  total. 

The  relation  which  hydrographic  conditions  bear  to  the 
copepodan  population  may  be  inferred  in  part  from  the  comparison 
of  years  given  above,  and  from  the  following  table,  in  which  are 
given  the  average  number  of  Copepoda  per  m.3  and  the  total  monthly 
movement  in  river  levels  in  July-December,  1897  and  1898. 


July 

August 

September 

1897 

1898 

1897 

1898 

1897 

1898 

Average    Copepoda 
per  m.3  

81,543* 
5.2 

7,720 
7.4 

121,070 
2.6 

11,080 

7.5 

261,387 
0.6 

36,920 
6.2 

Total  movement  in 
levels   in  ft 

October 

November 

December 

1897 

1898 

1897 

1898 

1897 

1898 

Average  Copepoda 
per  m.3  

128,093 
0.6 

28,285 
3.9 

49,240 
2.2 

10,692 
2.6 

15,740 
0.5 

7,908 
2.4 

Total  movement  in 
levels,  in  ft.  ... 

With  a  total  movement  of  1 1.7  ft.  in  July-December  in  1897  and 
nearly  three  times  as  much  (30  ft.)  in  1898,  we  find  copepodan 
population  falling  off  to  less  than  one  sixth  that  of  the  more  stable 
year. 

Of  the  total  Copepoda  in  our  records  for  1894-1899,  78  per  cent, 
are  nauplii  of  Cyclops  and  Diaptomus,  13  per  cent,  are  immature 
Cyclops,  and  the  remaining  9  per  cent,  are  HarpacticidcB,  Diapto- 
mus,  and  adult  Cyclops.  Of  the  twelve  forms,  Cyclops  viridis  var. 
insectus  is  the  most  important  quantitatively,  and  includes  one 
fourth  of  the  total  adult  copepodan  population,  exceeding  the 
next  in  importance,  C.  viridis  var.  brevispinosus,  by  over  threefold. 


261 

The  following  forms  are  of  numerical  importance  in  the  order 
named :  C.  bicuspidatus ,  young  Diaptomus,  Cyclops  edax,  Diaptomus 
sicilbides,  D.  pallidus,  Canthocamptus  spp.,  and  Cyclops  albidus. 
Cyclops  prasinus,  C.  modestus,  C.  phaleratus,  and  C.  serrulatus  are 
also  found,  but  in  such  small  numbers  as  to  be  of  no  quantita- 
tive consequence. 

DISCUSSION    OF    SPECIES    OF    COPEPODA. 

Argulus  sp. — A  small  and  apparently  young  argulid  was  found 
in  the  plankton  on  August  10,  1897.  Members  of  this  genus  are 
abundant  upon  Amia  calva  and  both  species  of  Lepisosteus,  all  very 
common  fish  in  channel  waters. 

Canthocamptus  spp.,  including  C.  illinoisensis  Forbes. — Average 
number,  78.  Canthocamptus  was  found  in  the  plankton  in  every 
month  of  the  year  but  June.  The  percentage  of  collections  contain- 
ing Canthocamptus  is  greatest  (44  to  63  per  cent.)  in  March-May  and 
November,  and  the  numbers  per  m.3  are  highest  in  March-May, 
when  females,  females  with  eggs,  and  nauplii  all  occur  in  their 
maximum  numbers.  All  records  of  totals  in  excess  of  400  fall  in 
this  vernal  period  with  the  single  exception  of  one  collection  in 
August,  1897.  The  largest  number,  3,058  per  m.3,  was  found 
April  29,  1896. 

Canthocamptus  occurs  throughout  the  whole  seasonal  range  in 
temperatures,  with  smallest  numbers  and  least  regularity  during 
maximum  summer  heat  in  June- August.  It  is  thus  a  planktont 
of  the  colder  rather  than  the  warmer  part  of  the  year. 

The  relations  which  hydrographic  conditions  bear  to  the  occur- 
rence of  Canthocamptus  in  the  plankton  may  be  inferred  from  the 
fact  that  of  the  48  records  in  1894-1899,  24  were  made  in  rising 
flood  waters,  14  in  falling  flood  stages  within  a  few  days  after  the 
culmination  of  the  rise,  and  but  10  in  stable  conditions  or  in  declining 
levels  when  flood  waters  of  recent  origin  did  not  fill  the  channel. 
From  these  facts  it  seems  probable  that  Canthocamptus  is  in  the 
main  adventitious  in  the  plankton  from  its  normal  habitat  in  the 
slime  at  the  bottom  and  margins  of  the  river  and  its  backwaters. 

Over  88  per  cent,  of  the  total  Canthocamptus  recorded  in  the 
plankton  consists  of  nauplii.  It  may  be  that — as  is  the  case  with  the 
young  Ostracoda — they  enter  the  area  of  the  plankton  more  readily 
than  the  adults.  Adults  were  found  in  the  plankton  only  in 


262 

November-May;  females  with  eggs,  only  in  February- April ;  and 
a  female  with  attached  spermatophore,  in  March.  Nauplii  appear 
in  greatest  numbers  in  April-May,  attaining  2,862  perm.3  April  24, 
1896,  but  they  rarely  rise*  above  400  per  m.3  outside  of  this  vernal 
period,  and  are  found  only  in  very  small  numbers  in  December- 
March.  It  appears  from  our  data  that  the  breeding  season  is  prin- 
cipally in  April-May. 

Cyclops  albidus  Jurine. — Average  number,  113;  in  1897,  136;  in 
1896,  33;  and  in  1894,  but  10.  A  discussion  of  the  variation  and 
synonymy  of  this  species  has  been  published  by  E.  B.  Forbes  ('97). 
The  species  is  numerically  least  important  of  the  dominant  members 
of  the  genus  in  our  plankton.  It  was  recorded  in  all  months  but 
December  and  February,  but  its  season  is  practically  confined  to 
April-October,  the  only  exceptions  being  three  records  in  small 
numbers  in  January,  March,  and  November,  and  two  of  larger 
numbers  (300  and  200)  in  the  higher  temperatures  of  the  delayed 
autumn  of  1897.  There  is  a  tendency  toward  a  summer  minimum 
in  June- July,  with  pulses  of  greater  amplitude  in  April-May  and 
again  in  August-October.  In  these  months  the  percentage  of 
collections  containing  C.  albidus  is  highest,  being  respectively  55, 
50,  38,  56,  and  53  per  cent.,  and  these  are  the  only  months  in  which 
the  numbers  per  m.3  rise  above  600.  The  highest  numbers  recorded, 
2,862  and  2,400,  occurred  respectively  on  April  24,  1896,  and 
October  5,  1897. 

Although  C.  albidus  is  found  in  the  extremes  of  temperatures, 
it  shows  a  decided  increase  after  temperatures  pass  60°  in  the 
vernal  rise,  and  falls  off  immediately  after  the  autumnal  decline 
passes  this  point.  With  high  temperatures  continued  into  October, 
in  1897  we  find  it  continuing  in  larger  numbers.  On  the  other 
hand,  during  maximum  summer  heat  (about  80°)  numbers,  as  a 
rule,  fall  below  300  per  m.3  The  temperature  optimum  thus  appears 
to  be  in  the  neighborhood  of  70°.  The  three  greatest  pulses  re- 
corded, occur  respectively  on  April  24,  1896,  at  72°;  on  April  26, 
1898,  at  57-°,  and  on  October  5,  1897,  at  71°. 

The  numbers  are  too  small  to  exhibit  very  clearly  the  phenome- 
non of  recurrent  pulses,  though  the  vernal  and  autumnal  pulses  are 
usually  well  defined,  and  in  the  stable  conditions  of  1897,  August, 
September,  October,  and  November  pulses  may  be  traced. 


263 

Hydrographic  conditions  appear  to  affect  C.  albidus  as  they  do 
other  Entomostraca.  In  July-December,  1897,  in  stable  low  water 
the  C.  albidus  population  exceeds  by  over  threefold  that  of  these 
months  in  1898. 

Of  the  totals  of  all  records  in  1894-1899,  74  per  cent,  are  fe- 
males,— 4  per  cent,  with  eggs  and  70  per  cent,  without, — and  the 
remaining  *26  per  cent,  are  males.  Immature  forms  and  nauplii 
were  not  distinguished  from  those  of  other  species.  Egg-bearing 
females  were  recorded  only  in  May  and  August-October,  at  times 
of  maximum  pulses.  Over  82  per  cent,  of  the  males  were  found 
in  August-October — a  period  of  declining  temperatures  and  decreas- 
ing food  supply. 

This  is  a  widely  distributed  species,  though  it  seems  generally 
to  be  present  in  relatively  small  numbers  in  the  plankton.  It  occurs 
in  many  European  lakes.  Stenroos  ( '98)  finds  that  it  is  the  most 
abundant  species  of  Cyclops  in  Nurmijarvi  See,  occurring  in  both 
the  plankton  and  littoral  fauna  throughout  the  summer.  Scourfield 
('98)  finds  it  common  in  the  waters  of  Epping  Forest,  where  it  is 
perennial  in  ponds  and  small  lakes ;  and  Burckhardt  ( '00)  also  finds 
it  in  the  smaller  lakes  of  Switzerland. 

It  appears  to  be  more  generally  reported  from  European  streams. 
Thus,  Schorler  ( '00)  finds  it  to  be  rare  in  the  plankton  of  the  Elbe 
at  Dresden. in  May;  and  Fric  and  Vavra  ('01),  perennial  in  the 
littoral  fauna  of  the  same  stream  at  Podiebrad,  while  Volk  ('03) 
reports  it  in  the  plankton  at  four  of  seven  localities  examined  at 
Hamburg.  Meissner  ('02  and  '03)  finds  it  in  May-August  in  the 
Volga  at  Saratoff,  where  it  is  abundant  in  the  littoral  zone  or  among 
vegetation  and  in  quiet  backwaters. 

Under  a  variety  of  synonyms  this  common  and  variable  species 
has  been  reported  from  many  American  waters  by  Herrick  ('84) 
and  others.  It  was  described  by  Professor  S.  A.  Forbes  ('90)  as 
C.  gyrinus,  from  the  plankton  of  Lake  Superior.  With  the  exception 
of  Marsh's  record  ('95)  from  Lake  St.  Clair,  it  does  not  elsewhere 
appear  to  have  been  found  in  the  plankton  of  the  Great  Lakes. 
Marsh  ('93  and  '95)  finds  it  generally  in  the  plankton  of  smaller 
bodies  of  water  in  Wisconsin  and  Michigan,  and  E.  B.  Forbes  ('97) 
reports  it  as  generally  distributed  in  American  waters  of  a  permanent 
character.  Brewer  ('98)  reports  it  (as  C.  signatus)  in  the  vernal 
plankton  of  deep  pools  near  Lincoln,  Neb.  No  statistical 


264 

data  on  its  seasonal  distribution  are  given  by  any  of  the  authors 
cited. 

C.  albidus  appears  thus  to  be  adapted  to  both  the  littoral  and 
limnetic  areas,  but  seems  never  to  attain  great  numbers  in  the 
latter. 

Cyclops  bicuspidatus  Claus. — Average  number,  373;  in  1897, 
206;  in  1896,  145;  in  1895,  312;  and  in  1894,  only  2.  A  full  dis- 
cussion of  the  variation  and  synonymy  of  this  species  has  been 
published  by  E.  B.  Forbes  ('97). 

This  species  shows  sharply  marked  seasonal  limitations.  Every 
one  of  the  68  records,  with  the  exception  of  one  of  a  single  female 
found  September  30,  falls  within  November-May,  and  all  of  the  May 
records  were  made  in  the  delayed  low  temperatures  of  the  spring  of 
1898.  The  general  distribution  of  this  species  during  this  period 
is  indicated  by  the  high  percentage  of  collections  in  which  it  was 
found,  viz.,  63,  71,  67,  73,  93,  53,  and  40,  respectively,  for  November- 
May.  The  numbers  per  m.3  are,  however,  high  only  in  November 
and  April-May,  reaching  8,000  in  1895  and  1898  in  this  vernal  pulse, 
and  3,560  in  November,  1897,  in  the  autumnal  pulse.  In  Decem- 
ber-March numbers  do  not  rise  above  500  per  m.3  save  once  in 
December  and  on  March  24-30,  1896.  C.  bicuspidatus  is  thus  a 
winter  and  early  spring  planktont  in  channel  waters  of  the  Illinois. 

The  temperature  adaptations  are  exhibited  by  the  fact  that  only 
13  of  the  68  occurrences  are  in  temperatures  above  50°,  only  5  above 
60°,  and  but  1  above  70°— that  of  May  24,  1898,  at  73°.  On  the 
other  hand,  the  greater  developments  in  numbers  take  place  during 
these  higher  temperatures  of  50°-70°,  the  only  rises  above  1,000  per 
m.3  at  temperatures  below  50°  being  those  of  March  30  and  April  10, 
1896,  at  48°  and  46.4°,  and  of  November  15,  1897,  at  47°.  Mini- 
mum numbers  thus  prevail  below  45°,  and  the  temperature  opti- 
mum in  channel  waters  of  the  Illinois  appears  to  lie  near  60°. 

The  seasonal  routine  in  channel  waters  begins  with  the  appear- 
ance of  small  numbers  about  November  1 ,  with  an  occasional  pulse 
of  some  amplitude  in  that  month  followed  by  a  continuance  of  small 
numbers  through  the  minimum  temperatures  of  December-Feb- 
ruary, and  a  rise  with  the  temperatures  in  March  to  a  maximum 
vernal  pulse  toward  the  end  of  April  or  the  first  of  May,  and  a 
complete  disappearance  of  adult  individuals  after  temperatures  pass 
70°  during  May-October. 


265 

Stable  hydrographic  conditions  appear  to  favor  the  increase  in 
C.  bicuspidatus,  as  is  seen  in  the  large  pulse  of  November  15,  1897 
(3 ,560) ,  and  the  slight  pulse  (240)  during  declining  levels  in  February, 
1899. 

The  vernal  development  of  1898  (Table  I.)  is  distinctly  pulse-like, 
and  there  are  traces  elsewhere  of  similar  phenomena,  but  in  general 
the  numbers  of  C.  bicuspidatus  are  too  small  to  exhibit  clearly  the 
phenomenon  of  recurrent  pulses. 

Of  the  totals  of  all  individuals  recorded  in  1894-1899  I  find  that 
37  per  cent,  are  males,  16  per  cent,  egg-bearing  females,  and  47  per 
cent,  females  without  eggs.  Immature  forms  and  nauplii  were  not 
distinguished  from  those  of  other  species.  With  the  exception  of 
a  few  stragglers,  the  egg-bearing  females  were  limited  principally 
to  March-May.  In  exceptional  cases  the  males  greatly  outnum- 
bered the  females,  as  on  November  15,  1897,  when  the  ratio  was 
2,820  to  680. 

Though  apparently  widely  distributed,  this  species  does  not 
appear  frequently  among  the  planktonts  reported  from  European 
lakes.  Scourfield  ( '98)  reports  it  as  a  common  species  in  the  waters 
of  Epping  Forest  throughout  the  year  with  the  exception  of  a  period 
of  absence  or  depression  in  July-August,  and  Scott  ( '99)  finds  it  in 
shore  collections  made  in  various  months  of  the  year  in  Scottish 
lakes,  and  more  abundantly  in  the  warmer  months.  It  has  been 
reported  in  the  potamoplankton  in  Europe  only  by  Rossinski  ('92) 
from  the  Moskwa,  by  Zernow  ('01)  from  the  Schoschma,  and  by 
Volk  ('03)  from  but  one  of  seven  localities  in  the  Elbe  at  Ham- 
burg. 

In  American  waters,  on  the  other  hand,  C.  bicuspidatus  is  more 
abundant,  and  in  the  Great  Lakes  it  forms  a  very  important  part 
of  the  plankton.  Forbes  ('82)  finds  it  (as  C.  thomasi]  to  be  the 
dominant  Cyclops  in  the  summer  plankton  of  Lake  Michigan  and 
('90)  also  abundant  in  that  of  Lake  Superior.  Marsh  ('93  and  '95) 
finds  it  in  the  summer  plankton  of  the  Great  Lakes,  near  Charlevoix, 
in  Lake  St.  Clair,  the  Detroit  River,  and  Lake  Erie,  but  only  rarely 
and  in  small  numbers  in  the  smaller  bodies  of  water  in  Wisconsin 
and  Michigan.  E.  B.  Forbes  ('97)  extends  its  recorded  range  to 
Massachusetts  and  to  the  lakes  and  rivers  of  Wyoming,  and  states 
that  it  is  widely  distributed  in  America  and  occurs  in  large  ponds 
and  rivers.  Brewer  ( '98)  reports  it  in  the  vernal  plankton  of  deep 


266 

pools  near  Lincoln,  Neb.  None  of  the  investigators  quoted  give 
statistical  data  of  the  seasonal  limitations  of  C.  bicuspidatus. 

The  absence  of  this  species  from  the  summer  plankton  of  the 
Illinois  River  and  its  abundance  in  that  of  the  Great  Lakes  is  perhaps 
explained  by  the  temperature  conditions.  Surface  waters  in  Lake 
Michigan  are  reported  by  Ward  ('96)  to  range  from  62°  to  67° 
August  11-29,  while  deeper  waters  at  and  below  the  thermocline 
reach  a  minimum  of  42°.  The  warmest  waters  there  (62°-67°)  are 
thus  considerably  cooler  than  the  coolest  in  the  waters  examined 
by  us  (which  are  usually  above  70°  and  often  above  80°)  during  the 
months  in  which  C.  bicuspidatus  is  not  found  in  our  plankton.  That 
its  absence  is  not  due  to  sewage  contamination  in  low  water  which 
usually  prevails  during  the  warmer  months  is  shown  by  the  prompt 
reappearance  of  the  species  in  the  autumn;  as,  for  example,  in  1897, 
when  sewage  was  even  more  abundant  than  usual.  It  may  be  that 
temperature  is  also  one  of  the  factors  limiting  its  distribution 
elsewhere. 

Cyclops  edax  Forbes. — Average  number,  49;  in  1897,  194;  in 
1896,  159;  in  1895,  321 ;  and  in  1894,  187.  This  is  the  third  species 
of  Cyclops  in  numerical  importance  in  channel  plankton  of  the 
Illinois. 

With  the  exception  of  a  single  record  on  November  2,  1897,  all 
occurrences  of  this  species  in  channel  plankton  are  confined  to 
April-October,  and  all  but  9  of  the  48  occurrences  are  in  July- 
October,  and  32  of  them  in  July- September — the  period  of  maxi- 
mum summer  heat.  During  these  three  months  the  percentage 
of  collections  containing  C.  edax  is  highest  (44  to  75  per  cent.),  and 
they  are  the  only  months  in  which  the  C.  edax  population  rises 
above  1,200  per  m.3  in  channel  waters  excepting  a  single  instance  on 
October  5,  1897,  in  the  high  temperatures  of  that  delayed  autumn. 
In  other  months  the  records  are  all  below  800  and  generally  below 
400  per  m.3  The  highest  number  recorded  was  3,600  on  October 
5,  1897. 

The  seasonal  distribution,  with  maximum  numbers  in  July- 
September,  exhibits  a  temperature  adaptation  on  the  part  of  C.  edax 
to  maximum  summer  temperatures  (70°  to  80°)  in  channel  waters. 
An  examination  of  the  records  shows  that  only  13  of  the  48  records 
of  this  species  fall  in  temperatures  below  70°,  and  these  were  all  in 
the  months  of  April,  May,  September,  October,  and  November,  at 


267 

times  when  occurrences  were  scattering  and  numbers  few;  that 
is,  during  the  rise  or  decline  of  the  species  to  or  from  the  summer 
maximum.  Of  the  13  records  below  70°,  there  were  5  between 
60°  and  70°,  7  between  50°  and  60°,  and  but  1  below  50°.  Cyclops 
edax  in  channel  waters  of  the  Illinois  is  thus  stenothermic  in  narrow 
limits  near  the  maximum  temperatures  of  the  year. 

The  relation  which  hydrographic  conditions  bear  to  the  seasonal 
development  of  C.  edax  may  be  inferred  from  the  fact  that  the 
July-October  population  of  this  species  in  the  disturbed  waters  of 
1898  was  only  35  per  cent,  of  that  in  the  more  stable  months  of  the 
preceding  year. 

The  occurrences  of  C.  edax  take  the  form  of  pulses,  though  less 
distinctly  recurrent  and  less  clearly  denned  than  in  species  present 
in  larger  numbers.  Such  pulses  appear  in  July,  August,  and 
September,  1895,  and  in  August  and  October,  1897.  In  1898 
(Table  I.)  the  numbers  present  are  too  small  to  clearly  indicate 
recurrent  pulses,  though  suggestions  of  the  phenomenon  appear  in 
the  records.  In  general  these  pulses  tend  to  coincide  with  those  of 
other  Entomostraca. 

Of  the  totals  of  all  our  records  of  C.  edax  in  1894-1899,  60  per 
cent,  are  females  without  eggs ;  1 1  per  cent.,  females  with  eggs ;  and 
29  per  cent.,  males.  Young  and  nauplii  were  not  distinguished 
from  those  of  other  species.  Egg-bearing  females  were  found  in 
April  and  in  June-October,  but  in  greatest  numbers  in  July- August. 
Males  occur  in  June-November,  with  no  marked  predominance  in 
any  period. 

This  species  has  not  been  separated  from  C.  leuckarti  by  other 
investigators  of  the  plankton,  though  E.  B.  Forbes  ('97),  after  a 
careful  comparison  of  American  forms  with  C.  leuckarti  of  Europe, 
concludes  that  edax  is  specifically  distinct,  and  that  leuckarti  also 
occurs  in  American  waters,  though  apparently  not  in  numbers  com- 
parable with  those  in  European  waters.  C.  edax  appears  in  a 
measure  to  replace  it  in  our  plankton.  He  reports  it  as  widely 
distributed  in  American  lakes  and  streams  and  in  the  plankton  of 
our  Great  Lakes. 

Cyclops  leuckarti  Claus. — A  single  dead  specimen  was  recorded 
in  channel  plankton  August  26,  1898.  E.  B.  Forbes  ('97)  records 
it  from  the  Fox  and  Sangamon  (tributaries  of  the  Illinois) ,  from  the 
Illinois  and  Mississippi  rivers,  and  from  Quiver,  Flag,  and  Dogfish 


268 

lakes,  backwaters  of  the  Illinois  at  Havana.  It  is  not,  however,  at 
any  time  a  factor  of  any  importance  in  channel  plankton  of  the 
Illinois  at  Havana,  being  confined  to  the  spring-fed  lakes  or  those 
shaded  by  vegetation,  where  regions  of  lower  temperatures  may  be 
found. 

This  is  a  widely  distributed  form  in  the  plankton  of  European 
waters.  Stenroos  ('98)  finds  it  abundant  in  the  plankton  of  Nur- 
mijarvi  See,  Scourfield  ( '98)  reports  it  as  common  in  the  waters  of 
Epping  Forest  in  February-October,  and  Scott  ( '99)  as  rare  in  that 
of  Scottish  lakes.  Fuhrmann  ( '00)  states  that  it  is  always  rare  in 
Neuenburger  See  except  in  April,  and  is  absent  in  November- 
December,  while  Burckhardt  ('OOa)  finds  it  to  be  perennial  in 
Vierwaldstatter  See,  with  breeding  season  in  May-September  and 
maximum  in  August  or  September. 

It  has  been  generally  reported  from  European  streams.  Schorler 
( '00)  finds  it  in  the  Elbe  at  Dresden  in  May-October,  with  greatest 
numbers  in  July-September,  and  Volk  ('03)  reports  it  from  four  of 
seven  localities  in  the  same  stream  at  Hamburg,  though  Fric  and 
Vavra  ( '0 1 )  do  not  find  it  at  Podiebrad.  Zykoff  ( '03 ) ,  Zernow  ('01), 
and  Meissner  ('02  and  '03)  find  it  in  the  plankton  of  Russian  rivers. 
The  last  author  states  that  it  occurs  in  both  channel  plankton  and 
littoral  fauna  among  vegetation  where  breeding  females  abound 
during  the  maximum  in  May.  The  young  only  appear  in  the  chan- 
nel plankton. 

In  American  waters  this  species  has  often  been  held  to  include 
C.  edax,  and  the  data  here  quoted  from  Birge  and  Marsh  refer  to  the 
combined  species.  Marsh  ( '93  and  '95)  finds  it  generally  distributed 
in  the  lakes  of  Michigan  and  Wisconsin,  and  in  the  plankton  of 
lakes  Erie,  Michigan,  and  St.  Clair.  Birge  ('97)  finds  it  in  the 
summer  plankton  of  Lake  Mendota,  where  it  is  even  more  abundant 
than  C.  viridis  var.  brevispinosus. 

Cyclops  modestus  Herrick  was  recorded  in  channel  plankton 
only  in  November,  December,  and  March,  in  small  numbers  and 
isolated  occurrences  at  temperatures  of  41°  and  below.  E.  B. 
Forbes  ('97)  states  that  this  species  lives  in  shallow,  weedy  water, 
and  has  never  been  found  in  large  numbers,  though  widely  dis- 
tributed. On  account  of  its  relative  rarity  it  may  have  been  over- 
looked by  me  and  have  a  wider  seasonal  distribution  than  my 
scanty  data  indicate. 


269 

Cyclops  phaleratus  Koch  was  recorded  in  channel  plankton  only 
in  small  numbers  in  November-December,  1897,  at  minimum  tem- 
peratures. E.  B.  Forbes  ('97)  states  that  it  is  a  littoral  form, 
confined  to  marginal  vegetation. 

Cyclops  prasinus  Fischer. — Average  number,  2.  This  species 
occurs  sparingly  and  irregularly  in  September-March  in  channel 
plankton,  appearing  in  largest  numbers  in  the  early  autumn  of  1895 
and  most  continuously  in  the  winter  of  1898-99.  The  numbers  are 
always  small,  never  reaching  400  per  m.3,  and  in  12  of  the  17  records 
falling  below  100  per  m.3  The  percentage  of  collections  containing 
C.  prasinus  in  the  totals  rises  above  20  per  cent,  only  in  December 
(24  per  cent.).  The  seasonal  distribution  in  channel  plankton 
indicates  a  limitation  to  the  colder  part  of  the  year,  all  records  but 

5  being  below  40°.     Nevertheless,  in  September-October,  1895,  the 
species  was  recorded  in  56°-79°.     This  fact  and  its  relatively  small 
numbers   generally,   make   it   probable  that   inferences   from   our 
scanty  data  concerning  its  seasonal  distribution  can  not  be  con- 
clusive. 

Of  the  totals  in  all  years,  86  per  cent,  are  females  without  eggs, 

6  per  cent,  females  with  eggs  (found  in  February  and  November), 
and  8  per  cent,  males. 

E.  B.  Forbes  ('97)  finds  the  species  widely  distributed  in 
American  waters  from  the  Great  Lakes  to  roadside  pools.  Marsh 
('93  and  '95)  finds  it  (as  C.  fluviatilis}  in  the  larger  bodies  of  water 
in  Wisconsin  and  Michigan,  and  in  lakes  Erie,  Michigan,  and  St. 
Clair.  In  Green  Lake  he  ('97)  finds  it  to  be  the  most  abundant 
species  of  Cyclops,  and  perennial,  with  maxima  in  September- 
November.  His  statistical  data  exhibit  somewhat  irregular  numbers 
which  contain  suggestions  of  recurrent  pulses  such  as  appear  in 
our  records  of  other  species  of  Cyclops.  Brewer  ('98)  finds  the 
species  in  the  plankton  of  pools  near  Lincoln,  Neb. 

Cyclops  serrulatus  Fischer. — Average  number,  3.  This  species 
was  taken  sparingly  in  channel  plankton,  exhibiting  only  isolated 
occurrences  in  December,  January,  March,  and  May,  in  flood  waters 
at  temperatures  of  32°-75°.  It  is  much  more  abundant  in  Spoon 
River,  where  it  is  sometimes  the  dominant  species  of  the  genus, 
appearing  in  May-September,  and  in  small  numbers  in  colder 
months.  It  appears  to  be  adventitious  in  channel  plankton  of  the 
Illinois  River. 


270 

This  widely  distributed  Cyclops  appears  but  rarely  in  the  records 
of  the  plankton  of  European  lakes,  and  then  only  in  the  smaller 
ones.  Stenroos  ('98)  reports  it  as  abundant  in  the  littoral  zone  of 
Nurmijarvi  See;  and  Scourfield  ('98)  finds  it  perennial  and  the 
most  abundant  species  of  Cyclops  in  the  waters  of  Epping  Forest. 

On  the  other  hand  it  has  been  found  generally  in  the  plankton 
of  European  streams.  Zimmer  ('99)  finds  it  in  the  Oder,  and 
Schorler  ( '00)  states  that  it  is  abundant  in  April- June  in  the  plank- 
ton of  the  Elbe  at  Dresden ;  Fric  and  Vavra  ( '01)  find  it  only  in  the 
littoral  fauna  at  Podiebrad;  and  Volk  ('03)  in  the  plankton  in  four 
of  seven  localities  in  the  Elbe  at  Hamburg.  Sowinski  ( '88)  found 
it  in  the  plankton  of  the  Dnieper,  Rossinski  ('92)  in  that  of  the 
Moskwa,  Zykoff  ('00)  in  the  summer  plankton  of  the  Volga,  and 
Zernow  ('01)  in  the  winter  plankton  of  the  Schoschma.  Meissner 
( '02  and  '03)  reports  it  in  May- August  as  not  abundant  in  the  back- 
waters and  vegetation  of  the  Volga  at  Saratoff. 

In  American  waters  Marsh  ('93  and  '95)  finds  it  in  smaller  lakes 
of  Wisconsin  and  Michigan  but  not  in  the  Great  Lakes,  and  E.  B. 
Forbes  ('97)  states  that  it  is  one  of  the  most  common  and  widely 
distributed  species  in  American  waters.  It  appears,  however,  not 
to  be  quantitatively  an  important  element  in  lake  or  river  plankton. 
Brewer  ('98)  finds  it  to  be  the  most  abundant  vernal  Cyclops  in  the 
small  bodies  of  water  near  Lincoln,  Neb. 

Cyclops  viridis  Jurine. — A  synonymy  and  a  discussion  of  varia- 
tions in  this  the  dominant  and  most  variable  of  all  the  Cyclops  in 
our  channel  plankton,  has  been  given  by  E.  B.  Forbes  ( '97).  I  have 
grouped  the  individuals  in  our  plankton  under  two  varieties, 
brevispinosus  Herrick  and  insectus  Forbes.  The  two  varieties  inter- 
grade,  and  in  my  separation  I  have  followed  only  a  single  character 
readily  visible  without  dissection  or  manipulation,  namely,  the 
outer  terminal  spine  of  the  stylet,  which  is  short,  broad,  and  lance- 
shaped  in  brevispinosus,  and  more  spine-like  in  insectus.  Judging 
from  the  results  of  this  method  of  separation,  it  appears  that  this 
lance-shaped  spine  is  a  character  of  the  male  in  many  instances, 
though  not  found  in  all  males  or  limited  to  this  sex. 

Cyclops  viridis  var.  brevispinosus  Herrick. — Average  number, 
124;  in  1897,  447;  in  1896, 622;  in  1895,  850;  and  in  1894,68.  This 
form  occurred  in  all  months  but  January,  but  predominantly  from 
the  last  days  of  April  to  the  first  week  in  October,  the  percentage 


271 

of  collections  containing  brevispinosus  in  these  months  being  27, 
80,62,67,48,75,  and  5 9  per  cent ,  respectively ,  while  in  other  months 
it  does  not  rise  above  20  per  cent.  The  number  of  individuals  is 
also  greater  during  the  warmer  season.  No  record  between  October 
15  and  April  20  exceeds  200  per  m.3,  while  between  April  20  and 
October  15  the  pulses  often  culminate  at  3,000-5,000  per  m.3,  and 
over  98  per  cent,  of  the  total  individuals  were  recorded. 

This  variety  appears  throughout  the  whole  seasonal  range  of 
temperatures  from  summer's  maximum  to  winter's  minimum,  but 
predominantly  during  the  warmer  season.  Only  15  of  the  71 
occurrences  and  2  per  cent,  of  the  individuals  wrere  recorded  at 
temperatures  below  60°.  As  soon  as  the  vernal  rise  in  temperatures 
passes  50°-60°,  the  minimum  numbers  and  scattered  occurrences  of 
the  winter  months  give  way  to  a  vernal  pulse  of  considerable  mag- 
nitude in  April-May,  attaining  4,452  on  April  25,  1895,  and  4,960 
on  May  25,  1897,  but  only  2,600  on  June  7,  1898.  This  is  followed 
by  a  period  of  depression  in  July,  when  the  summits  of  the  pulses 
did  not  often  surpass  1,000  per  m.3  In  the  late  summer  and  autumn 
of  1895  and  1897,  and  to  a  less  extent  in  1896  .and  1898,  a  second 
period  of  maximum  pulses  appears,  attaining  9,711  September  12, 
1895,  and  4,800  October  5,  1898.  When  temperatures  decline  in 
September-October  below  50°,  this  variety  falls  at  once  to  minimum 
numbers. 

The  records  of  brevispinosus  in  channel  plankton  exhibit  some- 
what clearly  the  phenomenon  of  recurrent  pulses  whenever  collec- 
tions at  brief  intervals  make  it  possible  to  delimit  the  pulses.  Thus, 
in  1895  there  are  pulses  culminating  in  July,  August,  September,  and 
October;  in  1896,  in  April,  May,  June,  July,  August,  and  September; 
in  1898,  in  July,  August,  and  October;  but  in  1898  (Table  I.)  the 
numbers  are  too  small  to  exhibit  fully  the  phenomenon  of  recurrent 
pulses. 

The  relation  to  hydrographic  conditions  may  be  inferred  from 
the  fact  that  while  in  the  stable  conditions  of  July-October,  1897, 
pulses  culminated  at  800-4,800  per  m.3,  in  the  same  period  in  the 
disturbed  hydrographic  conditions  of  1898  no  pulse  rose  above  200 
per  m.3,  and  the  total  of  all  records  in  those  months  is  only  8  per 
cent,  of  that  in  1897.  Evidently  brevispinosus  does  not  thrive  in 
flood  waters. 


272 

The  surprising  fact  derived  from  the  examination  of  our  records 
of  this  variety  of  C.  viridis,  is  that  the  individuals  referred  to  it  are 
predominantly  of  the  male  sex.  Out  of  a  total  of  74,308,  64,883,  or 
88  per  cent.,  are  males,  8,542,  or  11  per  cent.,  females  without  eggs, 
and  only  883,  or  one  per  cent.,  egg-bearing  females.  In  so  far  as 
these  data  go,  they  indicate  that  this  so-called  species,  or  even 
variety,  of  C.  viridis,  in  so  far  as  it  is  based  on  the  lance-like  spine 
of  the  stylet,  is  not  well  founded.  This  is,  it  seems,  predominantly 
a  male  character,  though  not  exclusively  so,  since  females,  and  even 
egg-bearing  females,  are  found  which  exhibit  this  structure. 

C.  viridis  var.  brevispinosus  appears  to  be  confined  to  American 
waters.  Marsh  ('93  and  '95)  reports  it  from  the  larger  lakes  of 
Wisconsin  and  Michigan,  and  from  the  Great  Lakes,  except  Lake 
Michigan.  Birge  ('95  and  '97)  finds  that  it  is  the  most  abundant 
species  of  Cyclops  (except  in  summer,  when  C.  leuckarti  abounds) 
in  Lake  Mendota,  and  the  only  one  reproducing  under  the  ice.  His 
data  exhibit  a  major  pulse  in  May,  and  a  second  one,  of  less  ampli- 
tude, in  October,  with  slight  indications  of  recurrent  minor  pulses 
in  midsummer,  obscured  possibly  by  the  massing  of  his  data  in 
fortnightly  averages.  The  seasonal  distribution  in  Lake  Mendota 
is  thus  much  like  that  in  the  Illinois  River.  Marsh  ('97)  finds  the 
maximum  in  Green  Lake  in  June  at  68°-69°,  and  only  scattering 
occurrences  at  other  seasons.  E.  B.  Forbes  finds  this  variety 
widely  distributed  in  American  waters,  but  never  especially  abun- 
dant. 

Cyclops  viridis  var.  insectus  Forbes. — Average  number,  539;  in 
1897,  2,115;  in  1896,  949;  in  1895,  2,966;  and  in  1894,  905.  It  is 
thus  more  abundant  by  two-  to  threefold  in  the  stable  years  of  1895 
and  1897  than  in  the  flood-swept  years  of  1896  and  1898. 

This  variety  was  found  in  every  month  of  the  year,  though 
predominantly  in  April-October,  when  the  percentages  of  the 
collections  containing  it  were  respectively  64,  100,  85,  100,  100,  87, 
and  76  per  cent.  In  November-March  the  percentages  were  only 
44,  6,  17,  7,  and  13.  The  numbers  of  individuals  are  very  small, 
however,  from  October  1  to  April  20,  excepting  in  the  autumn  of 
1897,  when,  with  the  delayed  high  temperatures  and  the  great 
impetus  given  to  plankton  development  in  the  stable  conditions  of 
low  water,  the  maximum  pulse  of  all  our  records,  30,800  per  m.3,  was 
reached  on  October  5,  a  pulse  of  1,200  following  in  November.  With 


273 

these  exceptions  no  record  exceeding  600  per  m.3  was  made  between 
the  dates  named.  Between  April  20  and  October  1  the  minimum 
records  rarely  fall  below  600  per  m.3,  except  in  1898,  and  the  pulses 
often  culminate  at  2,000-8,000.  C.  viridis  var.  insectus  is  thus  a 
planktont  of  the  warmer  season,  and  its  seasonal  distribution  is 
strikingly  similar  to  that  of  the  so-called  var.  brevispinosus. 

This  form  occurs  in  our  plankton  throughout  the  whole  seasonal 
range  in  temperatures,  but  only  in  small  numbers  and  irregularly 
below  60°.  Only  21  per  cent,  of  the  collections  containing 
insectus  were  made  at  temperatures  below  60°,  and  these  contained 
less  than  3  per  cent,  of  the  total  individuals.  With  the  exceptions 
of  the  pulses  culminating  at  43°  November  23,  1897,  at  1,200  per 
m.3,  and  at  57°  April  26,  1898,  at  4,160  per  m.3,  no  development  of 
this  species  exceeding  600  per  m.3  occurs  below  60°.  All  pulses  of 
more  than  3,000  per  m.3,  excepting  only  the  April  pulse  of  1898, 
occur  at  temperatures  above  70°.  The  species  reaches  its  greatest 
development  in  channel  waters  during  the  period  of  maximum 
temperatures,  70°-80°. 

The  seasonal  distribution  of  this  form  shows  a  few  straggling 
individuals  in  November-March  during  temperatures  below  50°, 
and  a  meteoric  rise  to  a  vernal  pulse  in  April-May  as  this  tempera- 
ture is  passed  and  60°-70°  arrives.  This  is  followed  by  a  series  of 
recurrent  pulses,  often  of  considerable  amplitude,  through  Septem- 
ber or  until  temperatures  fall  below  60°,  as  in  October,  1897.  With 
falling  temperatures  the  drop  in  numbers  to  the  winter  minimum 
is  quickly  accomplished.  A  comparison  of  the  distribution  in  1897 
and  in  other  years,  shows  a  close  correlation  between  the  decline  in 
temperatures  and  the  falling  off  in  numbers  of  insectus. 

The  relations  which  hydrographic  conditions  bear  to  the  develop- 
ment of  insectus  in  channel  plankton  may  be  inferred  from  the 
hydrographs  on  Plates  IX.-XII,  Part  I.,  and  from  the  data  sum- 
marized in  the  following  table, — 1894  being  omitted  because  of  the 
incompleteness  of  the  seasonal  representation. 

In  1895  levels  were  low,  unusually  so  in  the  spring,  and  the 
flood-free  intervals  of  the  year  were  of  more  than  the  usual  extent. 
About  10  feet  of  the  total  movement  in  levels  (51.9  ft.)  is  found  in 
the  late  December  rise.  If  this  is  excluded,  the  total  movement 
falls  to  42  feet,  and  the  range  in  levels  to  6.5  feet.  Under  conditions, 


274 


Year 

Range  in 
levels,  in  ft. 

Total 
movement, 

in  ft. 

Average  height, 
in  ft.,  of  stage 
of  river 

Average  number 
of  insectus 
per  m.3 

1895  

12.2 

51.9 

3.61 

2,966 

1896  

10.1 

45.7 

6.98 

949 

1897    

14  3 

44.8 

6   90 

2,  115 

1898  

15.5 

67.2 

8.02 

539 

then,  of  lowest  levels,  least  range,  and  total  movement,  we  find  the 
largest  development  (2,966)  of  insectus  in  channel  plankton. 

In  1896  the  average  river  level  is  much  higher,  affording  in- 
creased current  and  more  silt.  A  series  of  recurrent  floods  also 
flush  the  channel,  though  the  total  movement  and  range  in  levels 
within  the  limits  of  the  year  are  not  greatly  increased.  Neverthe- 
less, the  changes,  which  appear  mainly  below  bank-height,  affect 
channel  plankton  profoundly,  and  the  production  of  insectus  falls 
to  949  per  m.3  In  1897  the  population  rises  to  2,115  per  m.3,  largely 
as  a  result  of  the  stable  conditions  of  flood-free  waters  at  low  levels 
and  with  slight  current  in  the  last  half  of  the  year.  In  1898  the 
total  movement  (67.2),  range  in  levels  (15.5),  and  average  stage 
(8.02)  reach  the  extremes  in  the  four  years  under  comparison,  and 
the  insectus  population  falls  to  the  lowest  level — 539  per  m.3 

A  detailed  comparison  of  the  July-November  period  of  the  two 
years  follows. 


Month  

July 

August 

September 

Year  

1897 

1898 

1897 

1898 

1897 

1898 

Total  movement  

5.2 
6.05 

5,093 

7.4 
5.70 

210 

2.6 
2.29 

2,030 

7.5 
3.66 

304 

0.6 
2.01 

2,275 

6.2 
4.44 

325 

Average  stage  

Average  number  of  C. 
viridis  var.  insectus.  .  . 

275 


Month  

October 

November 

Average 

Year  

1897 

1898 

1897 

1898 

1897 

1898 

Total  movement  

0.6 
2.01 

8,625 

3.9 
4.86 

200 

2.2 

2.82 

520 

2.6 
7.44 

68 

2.2 
3.04 

T.709 

5.5 
5.22 

221 

Average  stage  

Average  number  of  C. 
viridis  var.  insectus.  .  . 

In  1898,  with  two  and  a  half  times  the  movement  in.  levels  found 
in  1897,  the  development  of  insectus  attains  less  than  6  per  cent,  of 
the  numbers  reached  in  the  latter  year. 

The  occurrences  of  insectus  in  channel  plankton  exhibit  the 
phenomenon  of  recurrent  pulses  during  the  season  of  its  occurrence 
in  large  numbers  whenever  collections  are  sufficiently  frequent  to 
delimit  the  pulses.  Thus,  in  1895  there  are.  such  pulses  in  July, 
August,  September,  and  October;  in  1896,  in  April,  June,  July, 
August,  and  September;  in  1897,  in  July,  August,  September, 
October,  and  November;  and  in  1898,  in  April,  May,  June,  July, 
August,  and  September,  though  of  slight  amplitude  in  the  last  three 
months. 

Some  of  the  seeming  gaps  and  irregularities  in  the  series  of  pulses 
of  brevispinosus  and  insectus  will  be  eliminated  if  the  statistics  of 
the  two  forms  are  combined  in  a  single  series, — a  fact  which  lends 
support  to  the  view  that  the  two  forms  belong  to  the  same  species, 
and  are  parts  of  a  common  group  of  variable  organisms. 

Steuer  ('01)  concludes  from  his  examination  of  the  plankton 
of  the  Danube  at  Vienna,  based  on  19  (?)  collections  in  15  months, 
that  Cyclops  has  usually  two  maxima  and  two  minima  in  each  year, 
and  that  in  the  same  body  of  water,  owing  to  various  meteorological 
influences,  the  two  maxima  do  not  in  any  year  fall  near  each  other. 
The  more  extensive  data  at  my  command  show  the  limitations  of 
such  a  general  conclusion.  An  examination  of  the  records  of  indi- 
vidual species  of  Cyclops  and  of  the  total  Cydopida  in  our  waters, 
make  it  clear  that  the  major  pulses  may  follow  each  other  at  about 
a  monthly  interval.  For  example,  in  1897,  the  total  Cyclopida 


(19) 


276 

have  their  major  occurrences  in  our  records  as  follows,  the  pulses 
appearing  September  14  and  October  5 : 

July    30 8,080  Sept.  14 117,000 

Aug.  10 49,360  Sept.  21 15,260 

Aug.  17 17,120  Sept.  29 14,400 

Aug.  24 20,320  Oct.   5 101,600 

Aug.  31 67,200  Oct.  12 3,400 

Sept.  7 107,200 

Again,  in  1896,  the  two  major  pulses  of  the  year  are  on  June  19 
(928,984)  and  July  18  (563,815).  Steuer's  conclusion  seems  to  be 
founded  upon  insufficient  data,  and  can  not  have  general  applica- 
tion. 

Of  the  total  240,830  individuals  of  C.  viridis  var.  insectus  in  our 
records  in  1894-1899,  117,166,  or  49  per  cent.,  are  males;  109,460, 
or  45  per  cent.,  females  without  eggs;  and  14,204,  or  6  per  cent., 
females  carrying  egg-sacs.  If  the  brevispinosus  totals  are  included, 
the  percentages  change  to  42  per  cent,  of  females — of  which  37  per 
cent,  and  5  per  cent.,  respectively,  are  without  and  with  egg-sacs — 
and  58  per  cent.,  males.  The  apparently  high  proportion  of  males 
may  be  due  to  the  fact  that  in  the  enumeration  more  young  females 
than  males  were. included  in  the  "young"  Cyclops. 

The  egg-bearing  females  were  generally  more  numerous  in 
April- July.  No  marked  predominance  in  the  proportion  of  males 
appears  at  any  season  in  our  records. 

Cyclops  viridis  does  not  appear  extensively  in  the  plankton 
literature  of  European  lakes.  Stenroos  ( '98)  finds  it  not  rare  in  the 
littoral  fauna  of  Nurmijarvi  See.  Scourfield  ('98)  reports  it  as 
next  in  abundance  to  C.  serrulatus  in  waters  of  Epping  Forest,  wrhere 
it  is  perennial.  Scott  ('99)  finds  it  at  all  seasons  in  both  littoral 
and  pelagic  collections  in  Scottish  lakes,  and  Amberg  ('00)  lists  it 
for  Katzensee. 

It  appears  but  infrequently  in  the  investigations  of  European 
streams.  Neither  Schorler  ('00)  nor  Fric  and  Vavra  ('01)  report 
it  from  the  Elbe,  though  Volk  ( '03)  lists  it  from  six  of  seven  localities 
in  this  stream  at  Hamburg.  Sowinski  ( '88)  finds  it  in  the  littoral 
fauna  of  the  Dnieper,  and  Zykoff  ('03)  in  the  summer  plankton  of 
the  Volga,  though  Meissner  ( '03)  states  that  it  is  never  found  in  the 
plankton  of  that  stream  at  Saratoff,  being  confined  to  the  littoral 


277 

zone  and  to  vegetation.  No  statistical  data  concerning  its  seasonal 
distribution  are  given  by  any  of  these  authors,  though  Meissner 
states  that  it  reaches  its  maximum  in  May  in  the  Volga. 

In  addition  to  the  species  of  Cyclops  here  listed  for  the  channel 
plankton  of  the  Illinois,  E.  B.  Forbes  ('97)  records  in  May-Septem- 
ber, 1896,  C.  varicans  Sars  as  common,  and  C.  fimbriatus  var.  poppei 
Rehberg  and  C.  bicolor  Sars  as  rare. 

Owing  to  the  impossibility  of  separating  with  certainty  the 
nauplii  and  young  of  the  various  species  of  Cyclops  they  were  all 
recorded  together  under  the  head  of "  nauplii "  and  "  young  Cyclops." 
The  former  includes  also  the  nauplii  of  the  two  species  of  Diaptomus 
occurring  in  our  plankton. 

Young  Cyclops. — Average  number,  4,780;  in  1897,  16,035;  in 
1896,  10,196;  in  1895,  21,960;  and  in  1894,  5,960.  With  two  ex- 
ceptions in  January  and  February  they  occur  in  every  collection 
examined.  Numbers  are,  however,  at  a  minimum  in  November- 
March,  only  9  instances  of  more  than  1,500  per  m.3  appearing  in  our 
records  in  this  season.  With  the  exception  of  two  pulses  in  the 
autumn  of  1897,  and  two  in  this  season  in  1895,  all  pulses  of  an 
amplitude  exceeding  8,000  per  m.3  are  confined  to  the  interval 
between  April  20  and  October  1,  practically  to  temperatures  above 
70°.  They  also  exhibit  relations  to  hydrographic  conditions  of  the 
same  nature  as  those  found  in  case  of  the  adults  of  the  various 
species  of  Cyclops,  and  manifest  likewise  the  phenomenon  of  re- 
current pulses  (Table  I.).  The  totals  of  all  young  Cyclops  in  1894— 
1899  are  almost  five  times  those  of  all  adults  of  the  genus.  This 
ratio  gives  an  index  of  the  extent  of  the  decimation  by  enemies  and 
inimical  factors  of  the  environment  which  exists  after  the  nauplius 
stage  has  passed  and  before  that  of  the  adult  is  reached. 

Nauplii  of  the  Copepoda  (excluding  the  Harpacticidce) . — Average 
number,  36,707;  in  1897,  53,786;  in  1896,  24,560;  in  1895,  88,442; 
and  in  1894,  45,648.  Nauplii  were  recorded  in  all  collections  ex- 
amined with  but  two  exceptions.  As  in  the  case  of  the  adults  and 
young,  the  large  numbers  are,  however,  confined  to  the  warmer 
season  between  April  1 5  and  October  1 .  During  the  colder  months 
the  pulses  rarely  rise  above  20,000  per  m.3,  and  those  in  excess  of 
35,000  during  these  months  are  with  one  exception  confined  to  the 
delayed  high  temperatures  of  the  stable  autumn  of  1897.  During 


278 


the  warmer  season,  on  the  other  hand,  the  pulses  frequently  attain 
100,000  or  over. 

The  maximum  record  of  928,984  was  made  in  the  stable  low 
water  of  June  19,  1895.  All  large  developments  thus  lie  at  tem- 
peratures above  70°. 

The  nauplii  bear  much  the  same  relation  to  hydrographic  condi- 
tions as  that  found  in  the  adults;  for  example,  in  Cyclops  viridis. 
This  is  seen  in  the  fact  that  in  unstable  years  such  as  1896  and  1898 
the  numbers  are  on  the  average  only  28  and  68  per  cent,  of  what 
they  were  in  the  more  stable  conditions  of  1895  and  1897,  and  the 
average  monthly  population  in  July-December  in  the  unstable 
conditions  of  1898  is  only  18  per  cent,  of  that  in  the  same  months 
of  the  previous  year. 

The  relative  numbers  of  adult,  young,  and  larval  stages  of  the 
Cydopida  are  given  in  the  accompanying  table. 


Year 

Nauplii 

Young  Cyclops 

Adult  Cyclops 

No. 

Ratio 

No. 

Ratio 

No. 

Ratio 

1894  

456,483 
2,741,718 
1,451,524 
1,828,720 
1,908,780 
121,345 

38 
19 
17 
18 
30 
61 

59,598 
680,749 
428,211 
545,200 
248,576 
5,422 

5 
5 
5 
5 
4 
3 

11,726 
140,779 
84,786 
102,730 
62,735 

1 
1 
1 
1 
1 

1895  

1896  

1897  

1898  

1899  

Totals  

8,508,570 

21 

1,967,756 

5 

404,749 

1 

The  ratios  between  total  adult  and  young,  1  to  5,  are  fairly 
constant  in  the  different  years,  falling  to  1  to  3  in  January-March, 
1899,  and  to  1  to  4  in  1898, — a  year  in  which  the  colder  part  of  the 
year  was  most  fully  represented.  This  ratio  probably  represents 
more  truly  the  relationship  of  young  and  adult  in  the  total  yearly 
production.  The  ratios  of  adults  to  nauplii  in  the  several  years 
vary  considerably  from  the  totals  of  all  years  (1  to  21),  rising  to  1  to 


279 

61  in  winter  conditions  of  1899  (January-March),  and  falling  as  low 
as  1  to  17  in  1896.  This  was  a  year  of  recurrent  floods,  but  its  ratio 
is  in  sharp  contrast  with  that  of  1898  (1  to  30),  also  a  year  of  con- 
siderable hydrographic  disturbances  during  the  summer.  The  adult 
population  was  reduced  during  this  year,  and  especially  during  the 
summer  floods,  but  the  nauplii  do  not  fall  conspicuously  below 
those  of  other  years.  It  would  therefore  seem  that  the  deleterious 
action  of  flood  conditions  operates  more  effectively  upon  the  adult 
and  young  than  upon  the  nauplii.  This  fact  may  be  due  to  the 
relative  absence  of  spines  and  hairs  on  the  nauplii,  structures  which 
gather  silt  and  load  down  the  larger  forms  in  the  flood,  waters. 
The  greater  number  of  young  and  adults  in  1896  as  compared 
with  1898  may  be  due  to  the  more  gradual  rise  of  the  floods  of  the 
former  year  (see  PI.  X.  and  XII.,  Pt.  I.)  jand  the  proportionally 
greater  amount  of  silt  in  the  more  sudden  floods  of  the  latter. 

The  ratios  given  in  the  table  are  of  course  subject  to  the  error 
arising  from  the  uneven  seasonal  distribution  of  the  collections  in 
some  years,  and  to  that  arising  from  varying  location  of  the  collec- 
tions on  the  pulses,  especially  on  those  of  greatest  amplitude.  An  ad- 
ditional error  arises  from  the  leakage  of  the  smaller  nauplii  through 
the  meshes  of  the  silk  net.  I  have  found  on  experiment  that  they 
will  thus  escape  under  pressure  of  a  column  of  water  only  3-4  cm.  in 
height.  Their  dimensions  are  such  that  the  smaller  individuals 
can  pass  through  the  meshes  of  even  the  No.  20  silk.  It  seems 
probable  that  ratios  of  nauplii  to  adults  are  actually  greater  than 
our  records  indicate. 

The  relationship  which  the  pulses  of  nauplii  bear  to  those  of  the 
adult  Cydopidcz  may  be  inferred  from  an  examination  of  the  data 
of  Table  I.  An  analysis  of  the  seasonal  distribution  of  the  total 
young  and  adult  Cydopidce  and  of  the  nauplii  reveals  the  fact  that 
in  all  seasons  in  which  collections  at  approximately  weekly  intervals 
were  made,  their  pulses  coincide  in  a  majority  of  cases  in  their 
maxima,  and  when  the  coincidences  do  not  occur  the  maximum  of 
the  nauplius  pulse  appears  in  the  collection  of  the  week  following 
that  of  the  young  and  adult  Cydopidcs.  This  appears  less  constantly 
and  clearly  in  the  disturbed  hydrographic  conditions  of  1898  (Table 
I.)  than  in  the  records  of  more  stable  years. 

Apstein  ('96)  finds  that  nauplii  of  Copepoda  are  most  abundant 
wrhen  eggs  are  most  common,  and  that  this  bears  no  constant  relation 


280 

to  the  abundance  of  adults.  Our  collections,  extending  over  longer 
periods  and  being  at  briefer  intervals,  indicate,  however,  that  this 
relation  does  exist.  As  above  stated,  the  larvae  are  most  abundant 
at  or  shortly  after  the  times  of  greatest  abundance  of  adults — that 
is,  the  maxima  of  the  recurrent  pulses.  Apstein  also  states  that 
reproduction  is  periodic  and  development  rapid.  Maximum 
numbers  are  reported  by  him  in  May  and  September. 

Cohn  ( '03),  on  the  other  hand,  maintains  that  the  "  innere  Logik" 
and  his  data  show  him  that  the  nauplii  reach  their  greatest  numbers 
just  prior  to  the  appearance  of  largest  numbers  of  young  and  adult 
Copepoda.  His  data  are  from  12  collections  between  May  1  and 
October  1 ,  and  favor  his  contention  in  2  out  of  3  cases  (of  maxima) , 
and  both  of  these  lie  in  collections  at  intervals  of  15  to  16  days.  In 
the  light  of  our  data  obtained  at  briefer  intervals  and  the  conclusions 
therefrom  that  the  pulses  of  larvae  tend  to  coincide  or  follow  at  a 
brief  interval  those  of  the  adults,  it  becomes  questionable  whether 
his  data  are  sufficient  for  his  conclusion.  His  logic  also  overlooks 
the  fact,  apparently,  that  smaller  numbers  of  larvce  might  lead  to 
coincident  maxima  of  grown  forms  during  a  period  of  abundant 
food,  on  which  all  pulses  must  be  based,  since  the  larval  stage  may 
be  at  such  times  a  brief  one  and  the  adult  a  relatively  longer  one,  and 
the  cumulative  effect  of  this  relationship  would  make  the  conditions 
shown  in  our  data  logically  possible.  Furthermore,  Cohn  used  a 
No.  12  silk  in  his  plankton  net,  and  this  allows  many  nauplii  to 
escape,  and  probably  accounts  for  the  fact  that  the  ratio  of  larvas 
to  grown  forms  in  his  figures  is  only  1.3  to  1,  while  in  our  records  it 
is  3.5  to  1.  The  discrepancy  arising  from  this  leakage  may  further 
tend  to  weaken  his  data  for  his  conclusions  concerning  the  relations 
of  larvas  and  adults. 

Steuer  ('01)  finds  that  the  nauplii  in  the  Danube  at  Vienna 
reach  maxima  in  June  and  in  August,  but  his  data  are  too  scattered 
to  fully  delineate  their  fluctuations.  Two  out  of  three  of  his  max- 
ima coincide  with  those  of  all  Cyclops,  and  the  third  antedates  it 
(monthly  intervals  of  collection),  as  in  Cohn's  data. 

Diaptomus  pallidus  Herrick. — Average  number  per  m.3,  11;  in 
1897,  367;  in  1896,  87;  in  1895,  152;  and  in  1894,  146. 

This  species  was  recorded  in  all  months  of  the  year  but  February, 
though  in  a  larger  percentage  of  the  collections  and  in  larger  numbers 
in  July-December.  Prior  to  this  season  the  percentage  does  not 


281 

rise  above  31  per  cent.,  the  occurrences  are  irregular,  and  the  num- 
bers are  small.  Thus  in  1896  and  1898,  years  of  numerous  winter 
and  vernal  collections,  there  were  but  4  occurrences  in  each  prior  to 
July  1 ,  and  all  but  one  of  these  was  of  numbers  less  than  100  per  m.3 
Only  12  of  the  72  occurrences  and  8  per  cent,  of  the  total  individuals 
were  recorded  in  the  first  and  less  stable  half  of  the  years.  In 
July-December  numbers  rise  in  feebly  outlined  pulses  which  attain 
at  the  most  800-2,400  per  m.3  The  percentage  of  collections  con- 
taining the  species  rises  to  33-75  per  cent.,  and  in  stable  autumns 
such  as  1895  and  1897  the  occurrences  are  but  little  interrupted.  In 
its  seasonal  distribution  in  channel  waters  it  is  thus  largely  confined 
to  the  last — and  more  stable — half  of  the  year. 

Its  relationship  to  hydrographic  conditions  here  suggested  also 
appears  in  a  comparison  of  the  yearly  averages  given  above.  The 
average  numbers  per  m.3  in  1896  and  1898,  87  and  11,  are  greatly 
exceeded  by  those  of  1895  (152)  and  1897  (367).  The  total  number 
recorded  in  July-December  in  1897  is  29  times  that  in  1898.  This 
well-defined  predominance  in  stable  seasons,  which  appears  also  in 
the  case  of  the  closely  related  D.  siciloides,  exceeds  that  of  the  other 
Entomostraca,  and  indicates  a  greater  sensitiveness  on  the  part  of 
these  species  to  the  deleterious  effects  of  flood  waters.  The  long 
antennas  and  great  development  of  the  feathering  of  the  caudal 
stylets  afford  a  large  area  for  the  attachment  of  the  silt  and  debris 
of  flood  waters,  and  accordingly  facilitate  the  destruction  or  removal 
of  Diaptomus  from  the  plankton  more  quickly  than  in  the  case  of 
Entomostraca  in  which  these  processes  are  less  developed — as  in 
Cyclops  or  Bosmina. 

The  numbers  of  individuals  are  too  small  to  delineate  accurately 
the  recurrent  pulses  which  are  suggested  in  the  data  of  distribution. 
In  the  autumns  of  1895  and  1897,  when  the  occurrences  are  most 
continuous,  the  larger  numbers  tend  to  fall  at  the  times  of  the 
maxima  of  pulses  of  other  Entomostraca.  There  is  no  marked 
limitation  placed  upon  this  species  by  the  seasonal  changes  in  tem- 
perature. It  is  found  throughout  the  seasonal  range  in  tempera- 
tures, though  numbers  are  slightly  smaller  in  channel  waters  in 
November-December.  Nevertheless  it  occurs  in  considerable  num- 
bers in  the  backwaters  in  breeding  activity  under  the  ice  at  mini- 
mum temperatures  in  December. 


282 

Of  the  total  individuals,  40  per  cent,  were  males;  45  per  cent., 
females  without  eggs;  and  15  per  cent.,  females  with  eggs.  The 
sexes  show  no  marked  or  constant  seasonal  differences  in  distribu- 
tion. Females  with  eggs  are  more  abundant  in  August-October, 
and  with  spermatophores  in  the  same  months.  Detached  sperma- 
tophores  were  found  until  December. 

This  species  is  stated  by  Herrick  ('84)  to  be  distributed  in  the 
entire  Mississippi  Valley.  Marsh  ('93)  finds  it  in  Wisconsin,  but 
it  appears  nowhere  in  the  plankton  of  the  Great  Lakes.  Brewer 
('98)  reports  it  in  the  backwaters  of  the  Platte  in  Nebraska,  and 
Schacht  ('97)  states  that  it  is  an  exceedingly  common  species  in 
central  Illinois,  and  that  it  has  been  reported  from  Wisconsin,  Ohio, 
and  Minnesota.  It  thus  appears  to  be  limited  to  the  shallow  and 
relatively  warm  waters  of  the  prairie  regions  of  the  Mississippi  basin. 

Diaptomus  siciloides  Lilljeborg. — Average  number,  10;  in  1897, 
350;  in  1896,  56;  in  1895,  282;  and  in  1894,  23.  As  will  be  seen 
on  comparison,  these  yearly  averages  are  very  similar  to  those  of 
the  preceding  species  with  the  exception  that  the  development  of 
D.  siciloides  is  about  twice  that  of  D.  pallidus  in  1895.  In  other 
particulars  its  seasonal  data  so  resemble  those  of  D.  pallidus  as  to 
make  their  discussion  in  large  part  a  repetition.  Its  seasonal- 
distribution  relations  to  temperature  and  hydrographic  conditions, 
breeding  season,  and  its  tendency  toward  a  pulse-like  recurrence  in 
coincidence  with  other  Entomostraca  are  all  very  similar  to  these 
features  in  D.  pallidus.  The  proportions  of  the  sexes  differ  slightly, 
the  males  being  less  numerous  (3 1  per  cent.)  and  egg-bearing  females 
more  abundant  (18  per  cent.)  than  in  the  previous  species. 

This  is  also  an  American  species,  reported  thus  far  only  from 
Lake  Tulare,  Calif.,  the  Illinois  River,  and  waters  of  Indiana  and 
Iowa  (Schacht, '97),  and  by  Brewer  ('98)  in  lakes  and  pools  of 
Nebraska.  It  is  thus  confined  largely  to  shoal  and  warm  waters. 

Diaptomus  spp.,  immature. — Average  number,  19;  in  1897,  560; 
in  1896,  158;  in  1895,  336;  and  in  1894,  120. 

The  immature  individuals  of  D.  pallidus  and  D.  siciloides  were 
not  distinguished  from  each  other  in  the  records.  Young  Diaptomus 
presumably  belonging  to  these  two  species  occur  in  every  month 
but  March,  though  but  10  of  the  74  records  were  made  in  January- 
June.  The  percentage  of  occurrences  and  the  numbers  per  m.3  are 
lowest  in  these  months,  not  rising  above  33  per  cent,  and  500  per 


283 

m.3  save  in  two  instances.  Occurrences  of  small  numbers  continue 
through  July,  but  from  August  1  to  October  15  appear  the  major 
pulses  of  the  year,  attaining  an  amplitude  of  1,000  to  8,800  per  m.3 
With  the  decline  of  temperatures  in  October,  numbers  fall  to  levels 
below  400  per  m.3,  with  one  exception  (December  14,  1897)  at 
700.  The  percentage  of  occurrences  is,  however,  high  (41  to  44 
per  cent.)  and  declines  only  to  33  per  cent,  in  January.  The  period 
of  greatest  numbers  of  young  thus  coincides  with  that  of  greatest 
abundance  of  adults,  and  lies  at  temperatures  of  70Vand  above,  in 
channel  waters. 

The  effect  of  hydrographic  changes  upon  the  occurrence  of 
young  Diaptomus  appears  in  striking  form  in  the  annual  averages 
above  quoted.  In  1898,  a  year  of  sudden  changes,  the  average 
per  m.3  is  only  19,  while  in  the  stable  conditions  of  the  previous  year 
it  is  560.  The  July-December  production  in  1897  is  28  times 
greater  than  that  of  1898.  In  1896,  a  year  of  recurrent  but  less 
sudden  floods, the  average  (158)  is  less  than  that  of  1895  (336),  a 
more  stable  year.  The  great  reduction  of  adults  noted  in  1898  and 
1896  is  thus  paralleled  by  an  even  greater  reduction  of  the  young. 

Osphranticum  labronectum  Forbes  occurs  in  the  plankton  of 
Quiver  Lake  in  small  numbers  (see  Schacht,  '98),  and  was  found 
once  in  channel  plankton  in  June,  1896. 

AMPHIPODA. 

Allorchestes  dentata  (Sm.)  Faxon. — This  is  an  abundant  littoral 
species  found  amid  vegetation,  especially  in  the  vegetation-rich 
backwaters,  such  as  Quiver  Lake.  It  was  not  often  found  in  channel 
plankton,  being  taken  only  in  the  summer  of  1895,  when  the  July- 
August  floods  carried  away  the  vegetation  which  had  accumulated 
during  the  antecedent  low  water. 

ARACHNIDA. 

ACARINA. 

In  vegetation-rich  backwaters  members  of  the  family  Hydrach- 
nid&  were  frequently  taken,  along  with  other  adventitious  or- 
ganisms, with  the  plankton.  In  channel  waters  they  are  less 
frequent,  and  are  represented  principally  by  Atax,  which  is  parasitic 


284 

in  great  numbers  (see  Kelly,  '99)  in  the  Unionida  which  are  found 
in  the  bottom  of  the  channel.  Occurrences  in  the  plankton  were 
limited  to  the  months  of  May-August,  and  may  be  due  in  part, 
especially  in  the  warmer  months,  to  the  release  of  the  parasites  by 
the  death  and  flotation  of  their  hosts.  Flood  waters  in  warm 
months  were  often  disastrous  to  the  Unionidce  because  of  the  load 
of  silt,  sewage,  and  industrial  wastes  which  they  carry  in  channel 
confines  at  the  lower  river  stages  often  prevailing  in  these  months. 
Other  small  aquatic  Acarina  were  also  present,  probably  adven- 
titious from  the  littoral  or  bottom  ooze.  With  two  exceptions  their 
occurrences  in  the  plankton  were  all  in  warmer  months,  April- 
September,  though  not  in  flood  waters.  During  the  period  of  the 
migration  of  waterfowl,  parasitic  Acarina  were  noted  in  plankton 
collections  in  a  few  instances. 

TARDIGRADA. 

Macrobiotus  macronyx  Duj. — Average  number,  11.  This  species 
is  found  principally  in  the  colder  part  of  the  year,  from  October  to 
May.  The  earliest  autumnal  record  was  October  30,  1895,  at  45°, 
and  the  latest  vernal  one,  May  1,  1896,  at  68.8°,  and  the  maximum 
number  (2,980  per  m.3)  was  recorded  on  April  10,  1&96,  at  46.2°.  Of 
this  number,  one  sixth  were  females  with  eggs.  Females  with  eggs 
were  also  found  in  November,  February,  and  March.  Because  of 
its  seasonal  distribution  it  is  found  principally,  though  not  solely,  in 
disturbed  hydrographic  conditions,  and  its  occurrence  in  the  plank- 
ton is  largely  adventitious. 

HEXAPODA. 

Owing  to  the  shoal  waters,  relatively  narrow  confines,  and  the 
hydrographic  fluctuations  in  our  fluviatile  environment,  the  aquatic 
insects,  both  larval  and  adult,  have  many  points  of  contact  with 
the  plankton.  They  constitute  a  large  element  in  the  total  volume 
of  the  animal  population  of  shore  and  bottom,  and  are  all  connected 
by  chains  of  food  relations,  more  or  less  complex  and  remote,  to  the 
plankton  organisms  or  their  sources  of  food.  With  the  single 
exception  of  the  larvae  of  Corethra  they  are  all  in  the  main  adventi- 
tious members  of  the  plankton  assemblage,  and  are  much  more 
abundant  in  the  vegetation-rich  backwaters  than  in  the  channel. 


285 

Since  the  aquatic  insects  of  these  collections  are  being  studied  by 
others,  with  reference  to  publication  in  this  Bulletin  (see  Hart, '95, 
and  Needham  and  Hart,  '01),  only  passing  notice  of  the  more 
important  representatives  appears  in  this  connection. 

EPHEMERIDA. 

Ephemerid  larvae,  as  a  rule  in  early  stages,  were  found  singly 
or  in  small  numbers  in  the  channel  plankton  in  the  warmer  months, 
April-October,  at  temperatures  above  56°.  Since  these  occurrences 
were  with  few  exceptions  in  stable  hydrographic  conditions,  it  seems 
probable  that  the  younger  larvae  of  this  order  may  adopt,  at  least 
temporarily,  a  limnetic  habit.  Specific  identifications  of  these 
larvae  were  not  made. 

HEMIPTERA. 

Corisa  (?)  sp. — Average  number,  37.  A  small  hemipterous 
larva  doubtfully  referred  by  Mr.  C.  A.  Hart  to  Corisa,  was  taken 
with  some  frequency  but  in  relatively  small  numbers  in  the  plankton 
during  the  summer  months.  Of  the  36  occurrences  27  fall  in 
June-August,  2  in  May  and  3  in  September,  2  in  January,  and  1  each 
in  October  and  November.  It  thus  appears  in  the  temperature 
extremes,  but  exhibits  a  great  predominance  in  the  season  of  maxi- 
mum heat.  There  is  no  marked  increase  in  its  frequency  or  numbers 
in  years  of  more  disturbed  hydrographic  conditions.  Its  numbers 
are  always  small  and  somewhat  erratic.  Adult  Corisa,  as  well  as 
many  other  aquatic  Hemiptera,  were  found  in  plankton  collections 
singly  and  infrequently. 

DIPTERA. 

This  group  of  insects  is  abundantly  represented  in  the  plankton, 
but  in  all  cases  by  larval  or  pupal  stages. 

Chironomus  spp.,  larval  stages. — Average  number,  124.  Larvae 
in  various  stages  of  development  from  that  immediately  after 
hatching  to  that  approaching  pupation  were  found  in  channel 
plankton.  They  occur  in  considerable  numbers  in  the  ooze  in  the 
river  bottom,  but  appear  to  abandon  the  limicolous  for  the  limnetic 
habit,  temporarily  at  least,  as  a  result  of  hydrographic  or  other 
disturbances.  There  is  evidence  from  their  relative  numbers  in 


286 

years  of  different  hydrographic  conditions  that  these  have  consider- 
able influence  in  bringing  them  into  the  plankton.  Thus  in  1897, 
in  stable  conditions,  there  were  only  5  occurrences  in  3 1  collections 
examined,  averaging  88  per  m.3,  while  in  1898,  in  more  disturbed 
conditions,  there  were  29  occurrences  in  52  collections,  averaging 
124  per  m.3  There  is  also  a  marked  seasonal  distribution.  The 
larvae  appear  in  the  plankton  in  March-December  through  the 
seasonal  extremes  of  temperature,  but  the  numbers  in  March  and 
November-December  are  always  small.  Only  15  per  cent,  of  the 
occurrences  and  5  per  cent,  of  the  individuals  were  found  at  tem- 
peratures below  45°.  The  percentage  of  occurrences  in  the  collec- 
tions is  highest  in  March-September,  the  percentages  being  53,  73, 
80,  47,  78,  52,  and  50,  respectively,  to  8  to  35  per  cent,  during  the 
remaining  months. 

Corethra  sp.,  larval  stages. — Average  number,  6.  These  semi- 
transparent  and  active  larvae  have  the  characteristics  of  limnetic 
organisms,  and  may  be  reckoned  among  the  autolimnetic  planktonts 
of  our  waters.  Because  of  their  activity ,  it  seems  probable  that 
they  escape  the  drawn  net, — especially  the  small  model  used  by 
us, — and  also,  because  of  their  negative  rheotaxis,  elude  the  suction 
of  the  plankton  pump  to  an  even  greater  extent.  Thus,  in  1895,  in 
net  collections,  there  were  8  occurrences  averaging  32  per  m.3  to  4 
in  1898,  in  pump  collections,  averaging  8  per  m.3  Corethra  larvas 
were  never  abundant  in  our  plankton,  probably  in  part  for  the 
reasons  just  cited.  With  two  exceptions  all  the  occurrences  lie  in 
the  period  of  maximum  temperatures  in  June-September,  7  of  the 
14  occurrences  and  one  third  of  the  individuals  being  recorded  in 
August. 

Dixa  sp.,  larval  stages. — Average  number,  8.  Larvas  were 
recorded  singly  in  scattered  occurrences  in  all  months  but  February 
and  October-December,  though  most  of  them  appear  during  maxi- 
mum temperatures. 

Larvae  of  Tanypus  and  Odontomyia  were  also  recorded  in  May 
and  June  in  isolated  occurrences. 

In  addition  to  the  larval  stages  of  these  aquatic  insects  there 
occurred  in  the  plankton  a  considerable  number  of  insect  eggs, 
principally  those  of  Diptera  and  Ephemerida.  These  were  generally 
isolated,  though  sometimes  fragments  of  the  egg-string  of  Chirono- 


287 

mus  appeared.  They  were  recorded  in  all  months  but  February 
and  December,  though  20  of  the  30  records  and  81  per  cent,  of  the 
individuals  appeared  in  May- August.  The  numbers  are  never  very 
large,  the  maximum  record,  5,424  per  m.3  on  June  29,  1894,  being 
due  to  a  number  of  fragments  of  egg-strings. 

MOLLUSCA. 
GASTROPODA. 

The  adults  and  young  of  many  of  our  aquatic  gastropods  have 
the  habit  of  gliding  on  the  under  side  of  the  surface  film  of  water,  and 
they  are  also  frequently  dislodged  from  their  foothold  on  aquatic 
vegetation,  and  thus  enter  the  habitat  of  the  plankton  temporarily. 
This  is  especially  true  in  vegetation-rich  backwaters.  The  smaller 
forms,  such  as  Ancylus,  Amnicola,  and  Planorbis  parvus  were  occa- 
sionally taken  in  the  summer  plankton  of  the  channel. 

LAMELLIBRANCHIATA. 

This  group  is  represented  in  the  plankton  by  the  larval  stages, 
or  glochidia,  of  the  Uniomdcz,  which  form  an  important  part  of  the 
bottom  fauna  of  the  stream  and  its  tributaries. 

Anodonta  corpulenta  Cooper. — Average  number  of  glochidia,  21. 
The  seasonal  distribution  of  the  glochidia  in  the  plankton  is  very 
well  defined.  With  but  two  exceptions  the  48  occurrences  all  fall  in 
October- April,  and  40  of  them  in  November-March.  The  occur- 
rences are  thus  during  the  period  of  minimum  temperatures ;  indeed, 
31  of  the  48  are  at  temperatures  not  exceeding  35°  in  surface  waters, 
and  only  9  are  above  45°.  The  earliest  autumnal  record  is  Septem- 
ber 30,  at  58°,  and  the  latest  vernal  one,  June  6,  at  79°.  Generally 
the  earliest  records  are  in  the  closing  days  of  September  or  the  early 
ones  of  October,  and  the  latest  records  are  about  the  first  of  April. 
The  occurrences  are  more  frequent  in  December-March,  the  glo- 
chidia appearing  in  64,  50,  53,  and  60  per  cent,  of  the  collections, 
respectively,  in  these  months.  Their  numbers  are  also  several 
fold  greater  at  this  season  than  in  the  earlier  and  later  months  of 
their  occurrence.  The  period  of  minimum  temperatures  is  thus 
the  season  of  greatest  discharge  of  glochidia.  The  numbers  are 
always  relatively  small,  520  on  December  28,  1897,  being  the  maxi- 


288 

mum  record.  Their  fluctuations  are  erratic,  and  show  no  apparent 
relation  to  hydrographic  or  other  environmental  changes. 

Lampsilus  anodontoides  (Lea)  Baker. — Glochidia  referred  with 
some  uncertainty  to  this  species  appeared  somewhat  irregularly  in 
the  plankton  in  small  numbers  in  September-December  and  again 
in  June-July.  The  seasonal  distribution  in  two  periods  suggests 
the  inclusion  of  two  species. 

Arcidens  confragosus  (Say)  Simpson. — Glochidia  of  the  type 
referred  by  Lea  to  the  old  genus  Margaritana,  and  presumably 
belonging  to  this  the  commonest  member  of  this  genus  (as  formerly 
understood)  in  our  locality,  were  taken  in  the  plankton  December 
18,  1895,  in  small  numbers. 

BRYOZOA. 

This  group  is  represented  in  our  plankton  by  the  floating  stato- 
blasts,  when  these  occur,  as  in  Pectinatella  and  Plumatella,  by 
detached  and  floating  fragments,  as  in  Urnatella,  or  by  natant 
colonies,  as  in  Lophopus  and  Cristatella.  Genera  such  as  Fredericella 
and  Paludicella,  whose  statoblasts  sink,  fail  to  appear  in  the  plank- 
ton, though  in  some  cases  they  may  be  abundant  in  the  bottom 
fauna.  The  Bryozoa  are  plankton  feeders,  and  play  an  important 
role  as  plankton  reducers  in  vegetation-rich  backwaters. 

DISCUSSION    OF    SPECIES    OF    BRYOZOA. 

Cristatella  mucedo  Cuvier. — This  species  was  found  in  the  back- 
waters in  summer  months,  especially  in  Quiver  Lake.  Statoblasts 
probably  referable  to  this  species  occurred  sparingly  in  May  and 
August. 

Lophopus  cristallinus  Pallas. — This  rare  bryozoan  occurred  in 
the  channel  plankton,  though  not  in  our  quantitative  collections,  in 
July,  1897,  in  that  part  of  the  channel  containing  the  discharge  from 
Quiver  Lake.  Small,  free-swimming  colonies  of  5-50  zooids  were 
taken  in  surface  waters. 

Pectinatella  magnified  Leidy. — Statoblasts  of  this  superb  bryo- 
zoan were  not  uncommon  in  the  backwaters,  and  were  seen  several 
times  in  the  vernal  plankton  of  the  channel.  The  large  floating 
colonies  are  found  near  the  surface  in  July-October  in  the  open 
backwaters,  and  more  rarely  in  the  river  itself.  The  translucent 


289 

gelatinous  coenoecia  are  spherical,  ellipsoidal,  or  often  somewhat 
flattened.  The  longest  diameter  of  these  floating  masses  often 
exceeds  30  cm. 

Plumatella  repens  L. — This  is  by  far  the  most  abundant  bryozoan 
in  our  locality,  being  found  everywhere  on  submerged  vegetation 
in  the  backwaters.  It  often  develops  with  surprising  rapidity  on 
the  submerged  stems  of  plants,  where,  as  in  1896,  summer  floods 
reinvade  the  vegetation-covered  margins  of  reservoir  backwraters. 
It  is  represented  in  the  plankton  by  its  floating  statoblasts.  Their 
seasonal  distribution  shows  some  correlations  with  temperature, 
hydrographic  conditions,  and  the  seasonal  cycle  of  the  parent 
organisms.  During  the  period  of  minimum  temperatures  (Decem- 
ber-February, inclusive)  they  are  relatively  rare  in  the  plankton, 
appearing  in  30,  8,  and  20  per  cent.,  respectively,  of  the  plankton 
catches.  They  are  rare  in  high-  as  well  as  low-water  conditions,  as, 
for  example,  in  the  floods  of  1895-96  and  1898,  when  they  appear 
in  but  one  of  15  collections.  With  the  rise  of  temperature  in  March 
they  occur  more  frequently,  as,  for  instance,  in  1898  (Table  I.),  and 
continue  during  the  run-off  of  the  spring  flood.  The  occurrences 
rise  in  March-May  to  60,  46,  and  50  per  cent,  of  our  total  collections 
in  these  months,  and  the  numbers  also  are  larger.  For  example,  in 
1898,  81  per  cent,  of  the  total  individuals  for  the  year  were  found 
in  these  months.  The  discharge  from  impounding  backwaters,  the 
principal  breeding  grounds  of  the  parent  organisms,  doubtless  tends 
to  increase  the  numbers  of  statoblasts  in  channel  plankton  during 
this  season.  During  the  remainder  of  the  year,  June-November, 
the  percentage  of  occurrences  again  falls  to  30,  50,  24,  32,  18,  and 
44  per  cent.,  respectively.  The  50  per  cent,  in  July  is  due  to  the 
summer  flood  of  1896.  If  this  year  is  omitted  the  record  falls  to 
33  per  cent.  The  large  percentage  for  November  is  probably  due  to 
the  predominantly  higher  levels  of  this  month,  to  the  invasion  of 
lake  margins  seeded  with  statoblasts,  and  to  the  increased  activity 
in  the  fishing  industry, which  tends  to  disturb  the  summer's  growth 
of  vegetation  in  tributary  backwaters.  The  relations  to  the  seasonal 
cycle  of  the  species  are  patent.  The  summer  months,  June- 
September,  are  the  season  of  growth  and  spread  of  the  parent 
organisms  and  of  the  formation  of  statoblasts,  especially  as  receding 
levels  expose  the  water  margins.  Hydrographic  or  other  disturb- 
ances tend  to  increase  the  number  of  statoblasts  in  the  plankton 


290 

until  minimum  temperatures  are  reached,  when  minimum  numbers 
appear  in  the  plankton.  As  temperatures  rise,  the  statoblasts 
tend  to  float  and  become  more  abundant  in  the  plankton,  as  a  result, 
perhaps,  of  the  physiological  and  accompanying  physical  changes 
in  the  contents  of  the  statoblast.  The  declining  phase  of  the  major 
flood  of  the  year  is  thus  the  period  of  greatest  flotation  and  dispersal 
of  the  statoblasts. 

Urnatella  graciUs  Leidy. — This  unique  species  is  found  in  some 
abundance  on  the  projecting  margins  of  the  shells  of  the  Unionida 
which  line  the  river  bottom  in  many  reaches  of  the  channel.  Small 
fragments  of  the  colonies  containing  only  several  polypides  were 
found  in  the  plankton  in  May-August  and  October.  The  earliest 
record  was  May  25,  and  the  latest,  October  25,  at  48.5°. 


THE  PERIODICITY  IN  THE  MULTIPLICATION  OF  THE  ORGANISMS  OF 

THE  PLANKTON. 

One  of  the  most  obvious  conclusions  brought  to  light  by  the 
detailed  study  of  the  volumetric  fluctuations  of  the  plankton  pub- 
lished in  Part  I.  of  this  report,  and  most  strongly  reinforced  by  the 
statistical  data  showing  the  fluctuations  in  the  numbers~of  the  indi- 
viduals of  the  various  species  and  in  the  sums  total  of  the  various 
biological  groups  represented  in  the  limnetic  fauna  and  flora,  is  that 
plankton  production  is  fundamentally  rhythmic  or  periodic  in 
character,  viewed  either  in  its  constituent  elements  or  as  a  whole. 
This  total  result  is  simply  the  sum  of  a  like  phenomenon  pervading 
more  or  less  completely  and  coincidently  the  reproductive  cycles, 
the  rise  and  decline  in  the  numbers  of  the  typical  constituents  of 
the  plankton.  The  exceptions  to  this  rhythm  are  usually  found  in 
those  organisms  which  are  adventitious  in  the  plankton  and  have 
their  centers  of  growth  and  distribution  in  other  regions  than  the 
open  water. 

Many  illustrations  of  this  periodic  movement  in  the  multiplica- 
tion of  organisms  of  the  plankton  have  been  cited  in  the  preceding 
pages  and  may  be  seen  in  the  accompanying  plates.  As  an  illustra- 
tion for  discussion  in  detail  we  may  take  the  pulse  of  July,  1898, 
shown  in  the  volumetric  data  of  Table  III.  and  Plate  XII.  of  Part  I. 
The  fluctuations  in  the  biological  population  during  this  period  are 
also  tabulated  in  Table  I.  of  this  paper,  and  graphically  presented 
in  Plates  II.  and  IV.,  which  exhibit  the  movement  in  the  totals  of 
the  Chlorophycecz,  Bacillariacece,  and  chlorophyll-bearing  Masti- 
gophora,  and  of  the  Rotifera  and  Crustacea. 

In  the  volumetric  data  the  pulse  rises  from  a  minimum  of  .14  cm.3 
per  m.3  on  July  5  to  a  maximum  of  ,88  cm.3  on  the  19th,  declining 
again  on  the  26th  to  the  second  minimum,  of  .67  cm.3  Its  duration 
is  thus  four  weeks  and  its  amplitude,  in  comparison  with  many 
other  pulses  in  the  records,  relatively  slight.  It  occurs  in  the  more 
stable  conditions  of  declining  river  levels  and  midsummer  tempera- 
tures. The  following  list  gives  the  names  of  the  more  or  less  typical 
planktonts  considered  in  the  discussion  of  this  pulse.  Others, 
largely  adventitious  or  insignificant  in  numbers,  might  be  added 

(20)  291 


292 

to  the  list.  Forms  whose  antecedent  minimum  does  not  fall  on 
June  28  or  July  5  are  designated  by  a  superior  1 ;  those  whose 
maximum  does  not  fall  on  July  19  or  26,  by  a  superior  2  ;  and  those 
whose  subsequent  minimum  is  not  on  July  26  or  August  2,  by  a 
superior  3.' 

The  component  forms  and  groups  are  Crenoihrix,  etc.1,  total 
Schizophyce®,  Microcystis  ichthyoblabe1 ,  total  Chlorophycea,  Actinas- 
trum  hantzschii,  Crucigenia  rectangularis,  Pediastrum  boryanum1'2'3, 
P.  pertusum2' 3,  Raphidium  polymorphum1 ,  Scenedesmus  genuinus, 
S.  obliquus,  S.  quadricauda,  Schroederia  setigera,  total  Bacillariacece1, 
Cyclotella  kuetzingiana,  Diatoma  elongatum1,  Fragilaria  virescens2, 
Melosira  granulata  var.  spinosa1,  M.  varians2,  Navicula  spp.,  Synedra 
acus,  total  Conjugates1,  Closterium  acerosum,  C.  gracilis,  total  Protozoa, 
total  Mastigophora,  Eudorina  elegans,  Euglena  acus,  E.  oxyuris, 
E.  viridis,  Glenodinium  cinctum,  Lepocindis  ovum,  Pandorina 
morum3,  Phacus  longicauda1-  2>  3,  P.  pleuronectes2'  3,  Platydorina 
caudata,  Pleodorina  calif  ornica,  Trachelomonas  acuminata1,  T. 
hispida3,  T.  volvocina,  total  Rhizopoda,  Difflugia  globulosa,  total 
Ciliata2,  Codonella  crater  a2,  Halteria  grandinella2* 3,  Tintinnidium 
fluviatile2'  3,  total  Rotifera,  total  Bdelloida1'  2,  total  Ploima,  Anurcsa 
cochlearis  and  var.  tecta,  eggs  of  A.  cochlearis  and  var.  tecta,  A. 
hypelasma,  Asplanchna  brightwellii1' 2>  3,  Brachionus  angularis  and 
var.  bidens,  eggs  of  B.  angularis  and  var.  bidens,  B.  bakeri  and  vars. 
duniorbicularis1 ,  melhemi,  and  tuber culus1' 2,  total  of  all  varieties 
of  B.  bakeri,  B.  budapestinensis,  B.  militaris1'  2,  B.  pala  and  var. 
amphiceros,  B.  urceolaris  var.  bursarius,  B.  variabilis2*  3,  Mastigocerca 
carinata1,  Monostyla  bulla,  Polyarthra  platyptera,  eggs  of  P.  platyp- 
tera2'3,  Rattulustigris2,  Synch<ztapectinatal,S.  stylata,  eggsofSynchceta1, 
Triarthra  terminalis2'  3,  Pedalion  mirum1'  3,  total  Entomostraca1'  3, 
total  Cladocera1'  2>  3,  Bosmina  longirostris1'  3,  Ceriodaphnia  scitula, 
Chydorus  sphcericus,Diaphanosoma  brachyurum3,Moina  micrura1'2'3, 
total  Copepoda  lj  2>  3,  Cyclops  viridis  var.  brevispinosus  and  var. 
insectus,  C.  edax,  young  Cyclops1,  nauplii  of  Copepoda1'  3. 

An  examination  of  the  preceding  list  and  of  the  qualitative  data 
of  Table  I.,  reveals  the  fact  that  71  of  the  more  typical  planktonts 
are  found  in  appreciable  numbers  in  the  plankton  during  this 
month.  To  this  number  we  may  add  6  immature  forms  separately 
listed  in  the  table  and  14  group  totals,  making  in  all  91  sets  of 
statistical  data  bearing  on  the  components  of  this  pulse.  An  analy- 


293 

sis  of  the  behavior  of  the  constituent  species  shows  that  43  of  the  7 1 
species  (including  varieties  and  forms),  4  of  the  6  immature  forms, 
and  10  of  the  14  group  totals  reach  their  greatest  amplitude  on  the 
19th,  coincidently  with  the  volumetric  maximum.  Thus,  in  all,  a 
total  of  57  out  of  91,  or  63  per  cent.,  of  the  sets  of  data  are  in  pre- 
cise agreement  as  to  the  time  of  maximum  development.  Fur- 
thermore, of  the  remaining  35,  there  are  10  culminating  in  the 
collection  prior  to  the  19th  (on  the  12th),  and  16  on  the  next  subse- 
quent one  (on  the  26th,)  in  all,  26  or  29  per  cent,  which  culminate 
on  immediately  contiguous  dates  of  examination.  This  leaves  a 
residuum  of  only  about  8  per  cent,  which  do  not  exhibit  precise  or 
substantial  agreement  as  to  the  time  of  maximum  development.  In 
the  matter  of  the  location  of  antecedent  and  subsequent  minima  the 
agreement  is  less  pronounced,  possibly  because  the  enumeration 
error  is  relatively  greater  in  the  case  of  minimum  numbers.  We 
find,  however,  that  65,  or  72  per  cent.,  of  the  antecedent  minima 
of  the  pulses  occur  on  June  28  or  July  5,  and  71,  or  79  per  cent.,  of 
the  subsequent  minima  are  on  July  26  or  August  2.  Nineteen,  or 
20  per  cent.,  of  the  antecedent  minima  are  on  July  12;  and  10,  or 
11  per  cent.,  of  the  subsequent  ones  are  on  August  12.  There  is 
thus  a  residuum  of  not  over  10  per  cent,  of  instances  where  the  data 
of  species  or  group  totals  do  not  coincide  or  approximate  to  this 
pulse,  as  described,  in  position  of  maximum  or  one  or  both  of  the 
limiting  minima.  Considering  the  necessarily  large  error  entering 
into  our  data,  it  is  not  surprising  that  exceptions  should  occur. 
Some  exceptions — as,  for  example,  that  of  Pediastrum  pertusum 
(Table  I.) — are  plainly  not  due  to  insufficient  data,  but  are  appar- 
ently normal  dislocations;  that  is,  the  rhythm  of  this  species  at  this 
time  is  not  in  harmony  with  that  of  the  majority  of  the  components 
of  the  plankton. .  But  this  is  only  a  temporary  derangement,  and 
is  not  the  habitual  relationship  which  movement  of  production  in 
Pediastrum  bears  to  that  of  the  plankton  as  a  whole.  So,  also, 
many  of  the  Entomostraca  are  much  delayed  in  the  culmination  of 
their  increase,  running  over  to  August  2  or  9,  while  the  most  of  the 
other  planktonts  culminate  on  July  19  or  26.  This  lag  on  the 
part  of  the  Entomostraca  is  not,  however,  habitual,  as  will  be  seen 
on  examination  of  Plates  II.  and  IV.  This  tendency  toward  a 
coincident  rhythmic  movement  in  production  on  the  part  of  the 
constituent  organisms  of  the  plankton  will  be  found  throughout  all 


294 

the  data  where  collections  are  of  sufficient  frequency  to  adequately 
delineate  the  curve  of  production,  that  is  from  July,  1895,  to  Oc- 
tober, 1896,  and  from  July,  1897,  to  March,  1899,  a  total  of  37 
months,  and  suggestions  of  a  like  phenomenon  appear  in  the  less 
complete  data  of  other  years.  The  degree  of  agreement  indicated 
in  the  pulse  of  July,  1898,  will  be  found, on  examination  of  the  data 
in  Table  I.  and  in  the  plates  of  this  paper,  to  vary  with  the  environ- 
mental conditions.  Times  of  rapid  change  in  hydrographic  condi- 
tions or  in  temperature  generally  show  less  agreement,  and  more 
stable  conditions  will  exhibit  an  equal  or  even  greater  uniformity 
in  the  prevalence  of  the  pulse-like  rise  and  decline  of  the  component 
organisms. 

In  order  to  show  the  course  of  these  recurrent  pulses  in  the 
chlorophyll-bearing  planktonts,  the  total  Chlorophycetz,  Bacil- 
lariacecE,  and  chlorophyll-bearing  Mastigophora  on  the  one  hand,  and 
of  the  Rotifera  and  Entomostraca  (''Crustacea"  of  the  plates),  I  have 
presented  the  data  graphically  on  Plates  I.-IV.,  and  in  the  table  on 
pages  296-299  have  drawn  up  a  list  of  the  pulses,  indicating  the 
dates  of  the  collections  which  in  the  main  enter  into  the  respective 
pulses,  and  the  dates  of  the  maxima  or  culminations  of  the  five 
groups  named.  Owing  to  the  irregularities  in  the  data,  there  are 
some  instances  in  which  several  possible  dates  might  have  been 
chosen.  Reasons  for  the  choice  are  in  several  important  instances 
given  in  the  foot-notes 'to  the  table. 

It  is  evident  from  the  data  here  presented  in  graphic  and  tabular 
form  that  the  pulses  of  the  five  groups  of  organisms  tend  in  the  main 
to  coincide.  This  is  shown  in  Plates  I.-IV.,  and  in  the  fact  that  the 
average  divergence  of  175  group  pulses  listed  in  the  table  is  6.4 
days,  or,  if  5  aberrant  instances  are  omitted,  only  4.8  days.  In  other 
words,  the  pulses  of  the  totals  of  the  5  groups  included  in  the  table 
culminate  on  an  average  within  an  interval  of  6.4  (4.8  in  170  cases) 
days.  The  average  of  the  extreme  limits  between  maxima  of 
group  pulses  in  the  36  periods  of  movement  listed  in  the  table  is 
11.7  days. 

It  is  apparent  that  the  pulses  would  be  more  completely  de- 
lineated by  collections  at  daily  intervals,  but  even  in  the  somewhat 
irregular  and  at  times  chaotic  data  here  presented,  the  evidence 
seems  conclusive  that  the  seasonal  production  of  the  dominant 
species  and  groups  of  planktonts  tends  to  fall  into  coincident 


295 

recurrent  pulses,  which,  in  turn,  are  the  cause  of  the  similar  and 
often  coincident  volumetric  fluctuations. 

Attention  should  be  directed  to  the  fact  that  without  any  im- 
portant exceptions  this  recurrent  movement  pervades  all  the 
organisms  of  the  plankton  which  are  eulimnetic, — such  as  Scenedes- 
mus,  Melosira,  Trachelomonas,  Codonella,  Synchtzta,  Daphnia,  and 
Cyclops,— and  often  those  which  at  certain  seasons  become  tempo- 
rary planktonts,  such  as  Difflugia  and  Hydra,  but  not  with  any 
regularity  the  tycholimnetic  organisms,  such  as  bdeltoid.  rotifers  or 
nematodes.  It  affects  the  more  highly  organized  Rotifer  a  and 
Entomostraca  with  slower  growth,  longer  life,  and  consequent 
greater  cumulative  function  as  well  as  the  algae,  diatoms,  and 
flagellates,  where  rapid  multiplication,  brief  existence,  and  non- 
cumulative  (in  the  individual)  function  prevail.  The  large  share 
which  the  young  (eggs  and  immature  stages)  play  in  the  pulses  of 
Rotifera  and  Entomostraca  will  be  seen  in  Table  I.,  and  repeated 
attention  has  been  called  to  this  in  the  discussion  of  species.  The 
prevalence  of  breeding  females  and  of  eggs  or  young  during  the  rise 
of  the  pulse,  and  of  eggless,  moribund,  or  dead  individuals  or  their 
skeletons  during  the  decline,  is  a  common  phenomenon  in  all  well- 
defined  pulses.  No  species  of  plankton  organisms  appears  to  escape 
the  operation  of  this  recurrent  movement  in  production. 

The  proportion  of  individuals  surviving  from  one  pulse  to  the 
next  is  subject  to  great  variation,  being  often  least  when  the  ampli- 
tude of  the  pulses  is  greatest,  and  largest  when  the  pulses  culminate 
at  slight  amplitudes.  As  a  result  of  periods  of  minimum  develop- 
ment, it  follows  that  the  possible  length  of  life  of  most  plankton 
organisms,  even  of  the  Rotifera  and  Entomostraca,  in  the  plankton 
must  fall  within  rather  narrow  limits  of  a  few  days  or  a  fortnight 
at  the  most.  Since  the  contrasts  between  minimum  and  maximum 
numbers  are  relatively  greater  among  the  chlorophyll-bearing 
organisms,  it  follows  that  the  survival  proportion  is  less  in  these 
groups. 

The  duration  and  amplitude  of  the  plankton  pulses  will  vary 
within  certain  limits  according  to  the  method  of  delineation.  The 
volumetric  minima  and  maxima  present  the  total  product  in  cubic 
centimeters,  and  the  pulses  thus  marked  cat  have  been  described 
in  Part  I.  They  may  also  be  delineated  by  statistical  data  of  the 
total  plankton  or  of  its  larger  groups  of  organisms,  or  by  the  domi- 


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some  obscurity  results  at  times  from  the  inclusion  of  unusual 
proportions  of  -an  adventitious  population  with  flood  waters.  The 
selection  of  particular  organisms  as  representative  is  also  subject  to 
some  error,  since  seasonal  changes  in  temperature  and  other  more 
subtile  causes  often  deflect  or  suppress  their  development.  The  totals 
of  the  ChlorophycecB,  Bacillariacece,  and  chlorophyll-bearing  Masti- 
gophora,  and  of  the  Rotifera  and  Entomostraca  (PL  I. -IV.)  probably 
give  as  complete  and  accurate  a  delineation  of  the  recurrent  pulses 
as  the  statistical  data  afford,  since  they  include  relatively  few 
adventitious  organisms,  cover  the  entire  year,  and  swamp  more  or 
less  completely  individual  and  temporary  divergences  of  particular 
species.  The  delineation  of  the  pulses  by  statistical  data  is  obviously 
more  significant  than  the  volumetric  method,  since  it  more  clearly 
presents  the  results  of  the  reproductive  processes  which  lie  at  the 
foundation  of  the  phenomenon  of  recurrent  pulses ;  and  this  method 
is  also  free  from  the  unavoidable  error  arising  from  the  presence  of 
silt  in  the  collections. 

The  interval  between  collections  introduces  an  error  of  consid- 
erable moment  in  any  effort  to  determine  with  accuracy  the  duration 
of  individual  pulses,  that  is,  the  length  of  time  between  their  minima 
or  maxima.  Daily  collections  would  render  this  feasible,  but  with 
an  interval  of  a  week  or  more,  not  only  the  duration,  but  in  some 
cases  the  probable  separation  of  the  pulses  and  location  of  their 
maxima,  is  to  some  undetermined  degree  obscured. 

The  duration  of  the  pulses  of  the  five  groups  of  plankton  organ- 
isms shown  graphically  on  Plates  I. -IV.,  in  the  case  of  all  chlorophyll- 
bearing  organisms  considered  as  a  whole,  is  in  29  out  of  36  instances 
between  21  and  35  days,  less  than  21  in  2  cases,  and  more  than  35 
in  5,  reaching  extreme  limits  of  14  and  49  days.  They  average 
30.25  days  between  minima  and  29.97  between  maxima. 

The  rotiferan  data  in  the  same  months  may  be  divided  into  36 
periods,  in  33  of  which  pulses  are  traceable.  The  duration  of  pulses 
between  minima  lies  between  2 1  and  3  5  d  ay s  in  2  3  of  the  3  6  instances , 
falls  below  21  in  5,  and  is  above  35  in  8.  The  extreme  limits  are 
14  and  49  days. 

In  the  case  of  the  Entomostraca,  where  also  the  pulses  are  obscure 
in  a  few  of  the  intervals,  we  find  that  22  of  the  36  are  between  21 
and  35  days  between  minima,  5  are  below  21,  and  9  are  above  35. 


301 

The  extreme  limits  are  12  and  49  days,  and  the  average  duration 
is  29.9  days. 

From  the  data  here  presented  it  is  evident  that  the  pulses  are  in 
the  main  from  3  to  5  weeks  in  duration,  averaging  approximately 
29  +  days — a  little  less  than  one  calendar  month. 

The  amplitude  of  the  pulses  is  affected  profoundly  by  seasonal 
and  local  influences,  such  as  the  factors  of  temperature  and  chemical 
constituents  of  the  water,  and  the  hydrographic  conditions.  These 
have  been  discussed  in  connection  with  the  volumetric-data  in  Part  I. 
and  in  the  discussion  of  species  in  the  first  part  of  the  present  paper. 
Rising,  or  even  uniform,  temperatures,  hydrographic  stability, 
decaying  vegetation  or  access  of  sewage  or  other  fertilizing  constitu- 
ents, all  serve  to  increase  the  amplitude  of  the  pulses.  Declining 
temperatures,  dilution  or  suspension  of  access  of  fertilizers,  competi- 
tion of  gross  vegetation,  access  of  flood  waters  and  increase  in 
current,  all  tend,  in  the  main,  to  depress  the  amplitude  of  the  pulses. 
The  duration  of  the  pulses  is  not,  however,  thereby  essentially 
modified,  though  a  tendency  to  override  subsequent  pulses  and 
partially,  rarely  wholly,  to  submerge  them  is  at  times  of  major  pulses 
often  apparent  in  the  data. 

The  cause  and  significance  of  the  phenomenon  of  recurrent  pulses 
is  not  clearly  and  unmistakably  evident,  owing,  on  the  one  hand,  to 
the  irregularity  of  the  data,  and,  on  the  other,  to  the  great  complex- 
ity of  the  problem,  especially  in  the  fluctuations  and  varying 
combinations  of  environmental  factors. 

The  plankton  method  itself  is  subject  to  great  errors,  but  these 
are  largely  distributed,  and  careful  examination,  especially  of  the 
matter  of  dilution  and  computation,  has  failed  to  reveal  any  probable 
or  even  possible  source  in  the  method  to  which  these  recurrent  pulses 
can  be  traced. 

It  is  not  impossible  that  the  rhythm  here  noted  is  merely  a 
chance  outcome  of  the  statistical  method  and  without  biological 
significance;  that  it  is  wholly  accidental,  the  resultant  of  the  con- 
flicting and  varying  factors  of  the  environment  and  not  predomi- 
nantly or  continuously  initiated  by  any  one  factor.  On  the  other 
hand,  its  nature,  as  we  have  described  it,  is  such  that  we  are  led  to 
look  for  some  factor  in  the  environment  with  which  this  rhythm  of 
repetition  in  growth  of  the  plankton  organism  might  be  correlated,  or 
to  some  internal  or  inherent  factor  within  the  organisms  constituting 


302 

the  plankton,  or  to  the  interaction  of  environmental  and  internal 
factors. 

That  there  is  a  periodicity  in  the  reproductive  processes  of 
organisms,  of  both  plants  and  animals,  is  generally  apparent.  We 
see  it  in  the  flowering  and  fruiting  seasons  of  the  phanerogams,  and 
in  the  breeding  seasons  of  many  invertebrates,  of  mollusks  and 
insects,  and  of  the  vertebrates  generally, — of  fishes,  amphibians, 
reptiles,  birds,  and  most  mammals.  Fluctuations  in  environmental 
conditions,  notably  in  food  and  temperature,  influence  these  re- 
productive processes.  The  phenomenon  of  rise  and  decline  of  the 
microscopic  population  in  laboratory  aquaria  is  likewise  an  illustra- 
tion of  the  periodicity  of  organisms,  but  usually  within  a  briefer 
interval  than  that  of  the  organisms  above  mentioned.  The  studies 
of  Maupas  ('88)  and  Calkins  ('02)  have  shown  that  even  in  the 
seemingly  uniform  conditions  of  the  laboratory,  the  reproduction 
of  the  ciliate  Protozoa  is  essentially  periodic. 

On  a  priori  grounds  it  seems  highly  improbable  that  in  the  case 
of  the  organisms  of  the  plankton,  internal  factors  should  determine 
the  coincidence  of  the  periods  of  growth  and  reproduction  in  several 
hundred  species.  While  it  is  not  impossible,  or  indeed  improbable, 
that  these  species  of  the  plankton  if  bred  in  pure  cultures  or  uniform 
environment  would  still  exhibit  a  periodic  reproduction,  it  seems 
highly  improbable  that  so  diverse  an  assemblage  of  algae,  diatoms, 
flagellates,  protozoans,  rotifers,  and  entomostracans  as  is  found  in 
the  Illinois  River,  would  exhibit  in  laboratory  cultures  under 
uniform  conditions  any  such  coincidence  in  the  location  and  duration 
of  their  pulses  as  is  found  in  the  waters  of  the  stream.  Whatever 
the  internal  factors  involved  in  the  growth  and  reproduction  of 
plankton  organisms  may  be,  it  is  patent  that  we  must  look  for  some 
environmental  factor  or  factors  lying  at  the  foundation  of  the 
coincidence  of  seasons  of  growth  and  reproduction  of  plankton 
organisms,  which  results  in  the  phenomenon  of  recurrent  pulses  in 
species,  groups,  and  volumetric  plankton. 

We  may  simplify  the  problem  somewhat  by  recognizing  at  the 
outset  the  importance  of  nutrition  in  supplying  the  basis  for  the 
periodic  growth  of  any  organism.  The  rotifers  and  entomostracans, 
at  least  the  limnetic  types,  depend  in  large  measure,  either  directly 
or  indirectly,  upon  the  synthetic  planktonts,  such  as  the  algae,  dia- 
toms, and  flagellates,  for  their  food.  Since  the  pulses  of  these  animal 


303 

forms  (cf.  Plates  III.  and  IV.  with  I.  and  II.)  coincide  with  or 
follow  shortly  after  those  of  the  synthetic  planktonts  on  which  they 
feed,  we  may  conclude  that  the  cause  of  the  periodic  movement  of 
these  animal  groups  lies  in  the  periodic  fluctuations  of  their  food 
supply.  In  the  causes  which  control  this  periodic  growth  of  the 
chlorophyll-bearing  organisms  will  be  found  the  solution  of  the 
general  periodic  phenomenon  in  plankton. 

This  rhythm  is  primarily  one  of  growth  and  reproduction,  and 
its  solution  must  be  sought  in  the  forms  of  matter  anctenergy  which 
affect  these  processes.  The  nutrition  of  the  chlorophyll-bearing 
organisms  is  drawn  from  matter  in  the  river  water.  The  analyses 
contained  in  Part  I.,  Table  X.,  and  graphically  presented  on  Plates 
XLIII.  to  XLV.  trace  the  seasonal  fluctuations  in  the  nitrates— -one 
of  the  important  constituents  of  plant  food.  Neither  in  the  seasonal 
curves  of  this  or  other  forms  of  nitrogen  delineated  in  the  plates  is 
there  any  such  rhythm  of  occurrence,  though,  as  has  been  pointed 
out  in  the  discussion  of  the  chemical  conditions,  there  are  instances 
of  apparent  correlation  of  plankton  and  nitrate  pulses.  They  occur 
at  irregular  intervals,  and  do  not  form  a  continuous  series.  That 
there  might  be  a  rhythm  in  the  utilized  nitrates  (the  analysis  repre- 
sents only  the  unused  residuum)  is  of  course  possible,  or  that  it 
might  occur  in  some  other  constituent  of  the  food  not  determined 
in  the  analysis  is  not  impossible,  but  we  have  no  evidence  of  its 
existence. 

The  chlorine  in  our  river  waters  is  a  fair  index  of  the  amount  of 
sewage  or  pollution  by  animal  wastes.  It  is  subject  to  considerable 
fluctuations,  resulting  in  part  from  dilution  by  floods  or  concentra- 
tion in  low  waters,  and  there  are  other  pulses  not  traceable  to 
hydrographic  conditions,  which  perhaps  result  from  industrial 
wastes.  These  fluctuations  in  some  instances  coincide  with  those 
of  the  phytoplankton  in  question,  but  the  instances  are  few  and 
the  correlation  is  incomplete.  Upon  investigation  I  find  that 
sewage  pumpage  at  Bridgeport,  which  discharged  the  sewage  of 
Chicago  River  into  the  Illinois  and  Michigan  Canal  and  thence  into 
the  Illinois  River,  was  practically  continuous,  and  could  not  produce 
the  rhythm 'in  question.  The  sewage  of  Peoria  has  a  much  more 
immediate  effect  upon  the  chemical  conditions  in  the  river  at 
Havana  than  has  that  of  Chicago.  The  sewers  of  this  city,  I  am 
informed  by  Mr.  H.  E.  Beasley,  City  Engineer,  are  flushed  as 


304 

follows :  "  The  method  used  is  that  of  flushing  with  a  hose,  a  crew 
of  men  being  kept  constantly  at  work,  taking  them  about  a  period 
of  three  weeks  to  cover  the  entire  system.  The  water  is  allowed  to 
run  through  a  fire-hose  at  each  point  for  a  period  of  about  ten 
minutes."  This  system  was  in  use  during  the  years  of  our  opera- 
tions, and  it  offers  no  occasion  for  the  periodic  pulses  in  growth  of 
the  organisms  in  question.  Investigation  of  the  discharges  of  dis- 
tillery and  cattle-yard  wastes  into  the  stream  has  not  revealed  any 
periodic  fertilization  of  the  river  waters  from  these  sources.  The 
available  data  thus  fail  to  exhibit  any  periodic  rhythm  in  food 
matters  in  solution  and  suspension  in  the  river  water  with  which 
these  pulses  of  chlorophyll-bearing  organisms  might  be  correlated. 

Frequent  reference  has  been  made  in  previous  pages  to  the 
appearance  of  pulses  upon  the  decline  of  floods.  Flood  waters  bring 
into  the  river,  as  shown  by  the  chemical  analyses,  large  quantities  of 
silt  and  organic  wastes  in  suspension  and  solution.  They  inundate 
great  tracts  of  fertile  territory  rich  in  vegetation,  and  thus  add  to 
the  available  sources  of  food  for  the  phytoplankton.  Decline  of  the 
flood  affords  time  for  decay  and  solution  of  some  of  the  food  matters, 
and  time  also  for  breeding,  and  its  run-off  adds  to  the  volume  of  the 
plankton  in  channel  waters.  A  comparison  of  the  hydrographs  of 
the  years  in  question  (Part  I.,  PL  X.-XIII.)  with  these  recurrent 
pulses  (PL  I.)  will  show  that  many  if  not  most  of  the  pulses  appear 
on  declining  flood  waters,  and  that  many  of  the  larger  ones  follow 
the  major  floods.  Closer  analysis,  however,  shows  that  there  are 
sometimes  two  pulses  of  chlorophyll-bearing  organisms  on  the 
decline  of  a  single  flood,  and  that  they  may  also  occur  upon  rising 
flood  or  even  in  its  entire  absence.  Floods  unquestionably  affect 
the  amplitude  of  the  pulses,  and  to  some  extent  modify  their  location. 
They  seem  inadequate,  however,  to  explain  their  recurrence  and 
their  tendency  toward  a  uniform  interval.  Minima  between  pulses 
also  recur  on  declining  floods. 

Energy  as  well  as  matter  is  necessary  for  the  growth  of  the 
phytoplankton,  and  its  source  is  primarily  the  radiant  energy  of 
the  sun.  A  plot  of  the  tri-daily  air  temperatures  at  Havana  for 
1894-1896  (Part  I.,  p.  478,  Fig.  C)  inclusive,  exhibits  many  irregu- 
larities, a  few  of  which  partake  of  the  nature  of  recurrent  pulses  at 
approximately  monthly  intervals,  but  they  are  too  few  and  too  irregu- 
lar to  be  the  basis  of  the  recurrent  growth  of  the  phytoplankton. 


305 

The  importance  of  light  for  the  photosynthesis  of  chlorophyll  - 
bearing  plants  is  unquestioned.  The  liberation  of  oxygen  by  the 
plant  declines  as  the  light  fades,  and  is  at  its  lowest  ebb  in  darkness. 
The  access  of  light  to  the  phy toplankton  is  limited  by  several  factors 
of  the  environment,  principally  by  silt,  which  increases  the  turbidity, 
and  by  clouds,  which  interfere  with  the  penetration  of  the  sun 's  rays. 
The  fluctuations  of  the  silt  are  chiefly  the  result  of  floods,  and,  as 
above  stated,  the  floods  do  not  exhibit  a  rhythmic  pulse  which  can 
be  correlated  with  that  of  the  phytoplankton ;  much  less  do  the 
periods  of  rising  water  which  are  most  silt-laden.  The  cloudiness 
of  the  sky  varies  greatly  at  different  seasons  of '  the  year,  being 
predominant  at  times  in  the  autumn  or  winter  months.  It  is  sub- 
ject to  pulse-like  occurrences  of  variable  duration,  but  an  examina- 
tion of  the  records  for  central  Illinois  for  the  years  under  discussion 
does  not  disclose  any  periodic  rhythm  which  can  be  correlated 
continuously  with  that  revealed  in  the  statistical  records  of  the 
growth  of  the  phytoplankton. 

Another  factor  of  the  environment  which  modifies  the  quantity 
of  light  which  impinges  upon  the  chlorophyll-bearing  organisms 
of  the  plankton  is  the  light  from  the  moon.  The  amount  of  light, 
both  absolute  and  relative,  derived  from  this  source  is  not  great. 
According  to  the  calculations  of  Zollner,  the  light  from  the  sun  is 
618,000  times  as  bright  as  that  from  the  full  moon.  In  the  pre- 
sent connection  it  is  only  important  to  know  whether  the  moon- 
light contains  an  amount  of  solar  energy  sufficient  to  appreciably 
affect  the  photosynthesis  of  the  phytoplankton.  The  amount  of 
such  energy  utilized  in  photosynthesis  is  relatively  a  small  propor- 
tion of  the  total,  so  that  there  is  a  possibility  that  moonlight  may 
contribute  to  the  process  to  an  appreciable  extent. 

This  matter  was  investigated  by  Knauthe  ( '98),  who  determined 
the  fluctuations  in  the  gaseous  contents  of  the  w*aters  of  carp  ponds 
rich  in  Euglena.  While  this  author  does  not  report  upon  the  plank- 
ton of  the  ponds  investigated,  it  seems  quite  probable  that  carp 
ponds  rich  in  Euglena  would  present  conditions  very  similar  to  those 
found  in  the  Illinois  River,  which  has  a  remarkably  well-developed 
Euglena  water-bloom,  and  abounds  also  in  carp. 

The  following  table  presents  the  results  of  his  work  bearing 
upon  the  point  in  discussion. 


55 

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306 


307 

The  amount  of  oxygen  present  in  the  water  in  the  dark,  or  on 
dark  nights,  is  reported  as  0.20,  0.25,  and  0.27  cm.3  per  100  cm.3  of 
water.  In  bright  sunlight  in  the  laboratory,  and  with  the  unusual 
abundance  of  Euglena  due  to  the  collection  of  the  water  sample 
from  the  region  of  the  water-bloom,  it  rises  to  2.05  cm.3  In  the 
case  of  the  Spandauer  samples  it  rises  from  0.25  in  the  dark  to  1.15 
(an  increase  of  0.90  cm.3)  after  "long"  exposure  to  bright  sunlight 
in  the  laboratory.  The  oxygen  in  this  water  at  11 :00  p.  m.,  after 
exposure  to  moonlight,  amounted  to  0.45,  or  0.20  cm.3  more  than 
was  found  in  control  water  kept  in  the  dark.  In  this  instance  the 
apparent  increase  due  to  moonlight  is  2/9  of  that  due  to  sunlight.  In 
the  case  of  the  moonlight  the  analysis  was  made  at  11:00  p.  m., 
after  not  more  than  three  hours '  exposure.  The  moon  was  not  at  its 
greatest  efficiency,  since  full  moon  occurred  four  days  prior  to  the 
date  of  analysis.  In  the  case  of  the  sample  exposed  to  the  sunlight 
the  analysis  was  made  at  4:00  p.  m.,  after  "langer  intens  Sonnen- 
schein."  It  would  seem  probable  that  the  effectiveness  of  moon- 
light in  comparison  with  sunlight  in  photosynthesis  by  the  phyto- 
plankton  here  indicated  (2  to  9)  is  below  the  possible  maximum  and 
also  above  that  of  the  average,  since  it  was  obtained  when  the 
moon  was  but  four  days  past  its  maximum  effectiveness. 

If  we  accept  Knauthe  's  data  as  sufficient  to  establish  the  effec- 
tiveness of  moonlight  in  increasing  photosynthesis,  and  thus  the 
growth  of  the  phytoplankton,  we  find  in  it  a  recurrent  factor  of  the 
environment  to  whose  influence  we  may  seek  to  attribute  the  rhythm 
of  growth  of  the  chlorophyll-bearing  organisms. 

On  Plates  I.  and  II.  I  have  plotted  the  seasonal  distribution  of 
the  totals  of  the  Chlorophycece,  of  the  Bacillariacecz,  and  of  the 
Mastigophora  from  July,  1897,  to  April,  1899,  and  have  indicated 
the  times  of  full  moon  throughout  this  period  by  marks  at  the  bot- 
tom of  the  diagram.  The  diagram  shows  clearly  the  occurrence  of 
these  recurrent  pulses,  their  approximation  in  the  three  groups  of 
chlorophyll-bearing  organisms  upon  the  same  or  adjacent  dates,  and 
the  occurrence  of  their  maxima  in  some  cases  at  the  time  of  full 
moon  or  within  an  interval  of  ten  days  thereafter. 

In  the  table  which  follows,  I  have  given  the  data  bearing  on  the 
pulses  of  the  total  of  all  chlorophyll-bearing  organisms  from  July, 
1895,  to  October,  1896,  and  from  July,  1897,  to  March,  1899,  inclu- 
sive, 36  months  in  all,  stating  the  location  of  the  pulse  as  determined 

(21) 


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310 

in  most  cases  by  the  delimiting  minima,  the  interval  between 
maxima  and  that  between  minima,  the  date  of  the  maximum,  the 
deviation  of  the  beginning  and  of  the  maximum  of  each  pu]se  from 
the  day  of  full  moon,  the  deviation  of  the  abscissa  of  the  center  of 
gravity  of  the  polygon  formed  by  the  plot  of  each  pulse,  and  the 
date  of  full  moon.  Deviations  prior  to  the  day  of  full  moon  are 
preceded  by  the  minus  sign. 

The  average  duration  between  minima  is  30.25  days  and  that 
between  maxima  is  29.97  days;  the  average  location  of  the  initial 
rise  of  the  pulse  is  5.1  days  prior  to  full  moon;  and  the  average  lags 
of  the  dates  of  maxima  and  abscissa  of  center  of  gravity  of  the 
polygon  of  occurrences  are  11  and  10.45  days,  respectively.  The 
probable  error  of  the  location  of  the  abscissa  of  a  single  pulse  is  ±  7.5 
days,  and  of  the  average  deviation  of  the  abscissa  only  ±  1.25  days. 

The  table  on  pages  296-299  shows  the  lag  of  the  maximum 
individual  pulses  of  Chlorophycece,  Bacillariacece,  chlorophyll- 
bearing  Mastigophora,  Rotifera,  and  Entomostraca.  The  average 
lag  after  the  day  of  full  moon  for  each  of  the  groups,  in  the 
order  named,  is  13.7,  14.8,  14.3,  13.1,  and  14.3  days,  re- 
spectively, with  a  grand  average  of  14.1  days  for  the  175  pulses 
listed.  Of  these  pulses,  135,  or  76  per  cent.,  culminate  prior  to  the 
third  week  after  the  date  of  full  moon,  and  94,  or  52  per  cent.,  in 
the  fortnight  between  7  and  21  days  after  full  moon.  The  averages 
and  percentages  given  in  this  paragraph  vary  but  slightly  from  the 
demands  of  chance  in  favor  of  a  hypothesis  that  the  pulses  tend 
to  culminate  in  a  particular  part  of  the  lunar  month,  though  the 
data  of  the  total  chlorophyll-bearing  organisms  given  above,  es- 
pecially the  deviation  of  the  abscissa  of  center  of  gravity  of  the 
polygon  of  their  occurrences,  point  in  the  direction  of  a  lunar  factor. 

There  is  no  doubt  of  the  fact  of  recurrent  pulses  and  of  their 
distribution  at  intervals  whose  average  approximates  that  of  the 
lunar  month,  though  their  correlation  with. any  particular  part  of 
the  month  is  in  no  way  constant  and  much  less  apparent.  It  would 
not  be  strange  that  the  duration  interval,  or  that  the  position  of 
maxima  and  minima,  should  be  subject  to  disturbance,  to  accelera- 
tion and  delay,  even  to  obliteration,  in  the  fluviatile  environment 
with  its  multitudinous  factors, — flood  and  drouth,  summer  and 
winter,  clear  and  turbid  waters,  bright  skies  and  overcast,  the  rise 
and  fall  of  nitrates  and  other  substances*n-solution  or  suspension, 


311 

the  fluctuating  access  of  sewage  and  industrial  wastes,  the  continuous 
current,  the  ever-shifting  population  and  the  never  ceasing  struggle 
for  existence  and  continuance  on  the  part  of  the  interrelated  organ- 
isms of  the  plankton  and  of  the  shores  and  bottom.  The  wonder 
is  that  any  single  factor  of  the  environment,  however  constant, 
could  make  any  orderly  impression  in  this  chaotic  situation. 

This  fact  that  the  average  interval  of  the  pulses  of  the  phyto- 
plankton  is  so  nearly  the  lunar  interval  would  seem  to  indicate 
some  causal  nexus  between  the  two  phenomena.  An  attempt  to 
correlate  the  plankton  pulse  with  any  particular  part  of  the  lunar 
month  is,  however,  less  conclusive.  The  interval  of  collection,  one 
week,  is  so  great  that  the  course  of  the  ptilse  can  be  traced  only 
approximately,  since  its  beginning,  maximum,  and  end  can  only, 
from  our  data,  be  located  at  one  of  these  intervals,  and  more  or 
less  distortion  results  therefrom.  Again,  the  large  error  in  the 
plankton  method  may  be  responsible  for  some  of  the  fluctuations 
in  the  data.  Still  more  potent,  probably,  are  the  various  factors 
of  the  environment  of  the  plankton  which  combine  with  the  lunar 
illumination  to  produce  resultants  which  divert  the  pulse  more  or 
less  from  the  course  which  the  undisturbed  lunar  factor  would 
cause  it  to  take.  Evidence  in  favor  of  this  view  appears  in  the 
fact  that  the  greatest  disturbances  in  the  rhythmic  sequence  of 
the  pulses  are  wont  to  occur  in  winter  months,  when  floods,  ice, 
and  cloudy  weather  tend  most  to  interfere  with  the  full  action  of 
the  lunar  factor,  while  the  correlation  of  full  moon  and  phyto- 
plankton  pulse  is  most  intimate  in  the  stable  conditions  of  summer. 
This  is  seen  in  the  fact  that  the  average  of  the  average  monthly 
lags  for  all  of  the  May- August  pulses  is  11.9  days,  and  for  the 
remaining  eight  months,  18.2  days. 

The  subject  here  presented  is  one  which  lends  itself  readily  to 
field  and  laboratory  experiment,  and  it  is  to  be  hoped  that  the  sug- 
gestions of  a  correlation  between  the  plankton  pulses  and  lunar 
cycle  here  made,  will  be  put  to  the  test  of  further  quantitative  and 
statistical,  as  well  as  experimental,  tests  in  controlled  environments 
where  the  disturbing  factors  of  the  fluviatile  environment  are  elimi- 
nated. 


GENERAL  CONSIDERATIONS  ON  SEASONAL  CHANGES.* 

It  follows  from  the  facts  set  forth  in  the  preceding  discussion  that 
in  general  each  month  of  the  year,  characterized  by  a  certain  range 
of  hydrographic,  thermal,  and  chemical  conditions,  and  of  illumi- 
nation, has  a  plankton  characterized  as  follows : — 

1.  There  is  a  certain   range   of   component  species,    some  of 
which  are  occasional  stragglers  and  others  more  or  less  uniformly 
present. 

2.  There  is  a  certain  range  of  numbers  of  individuals,  varying 
with  the  species  and  profoundly  affected  by  fluctuations  in  the 
environmental  factors,  which  change  the  proportions  of  the  various 
species  from  year  to  year.     These  proportions  vary  also  from  month 
to  month  and  constitute  one  of  the  main  elements  in  the  seasonal 
changes  of  the  plankton. 

3.  Transitions  from  month  to  month    are  most   profound  at 
seasons  of  greatest  environmental  change,  as,  for  example,  at  the 
times  of  vernal  increase  and  autumnal  decline  in  temperatures. 

4.  Seasonal  changes  in  the  plankton  follow  the  environmental 
changes  and  not  the  calendar.     Autumnal  plankton  is  found  when 
autumnal  temperatures  arrive. 

5.  In  the  main,  but  two  types  of  plankton  are  found  in  the 
Illinois  River — the  summer,  and  the  winter  assemblage.    The  vernal 
and  autumnal  types  are  only  transitions  between  the  two  when 
organisms  from  both  are  present.     The  winter  plankton  is  charac- 
terized by  a  small  number  of  species  peculiar  to  that  season,  and  a 
number  of  perennial  forms;  the  summer,  by  a  larger  number  of 
summer  organisms  with  the  perennial  types. 

LAKE  VERSUS  RIVER  PLANKTON. 

Is  the  plankton  of  streams  (potamoplankton)  different  from 
that  of  lakes  (limnoplankton)  and  ponds  (heleoplankton)  ?  This 
terminology,  introduced  by  Zacharias  ( '98  and  '98a),  seems  to  imply 
a  distinction  which  lies  not  only  in  the  differences  in  the  configura- 

*  The  detailed  discussion  of  seasonal  changes  in  the  plankton  is  deferred  to  a 
later  paper. 

312      • 


313 

tion  of  the  basin  and  in  the  matter  of  movement  in  the  water,  but 
also  in  the  constitution  of  the  plankton  itself.  The  examination 
of  the  plankton  of  the  Illinois  River,  and  of  its  backwaters  and 
tributaries,  has  shown  that  the  plankton  of  the  channel  is  not  im- 
mediately derived  from  the  tributaries,  but  comes  in  large  part 
from  the  impounding  backwaters,  and  at  low-water  stages  is  almost 
exclusively  indigenous  in  the  channel  itself.  Upon  the  basis  of  the 
data  from  the  Illinois  River  the  potamoplankton  is  distinguished 
from  the  other  types  named  by  the  following  characters  :— 

1.  It  is  a  polymixic  plankton.     This  is  due  to  the  mingling  of 
planktons  from  all  sources  in  the  drainage  basin,  especially  from 
tributary  backwaters,  and  the  consequent  seeding  of  the  channel 
waters  with  a  great  range  and  variety  of  organisms.    In  all  of  our  col- 
lections in  channel  waters  monotonic  planktons  can  scarely  be  said 
to  be  present.     The  nearest  approach  to  such  conditions  occurred 
at  low- water  stages,  when  channel  waters  are  most  fully  isolated. 

2.  It  is  subject  to  extreme  fluctuations  in  quantity  and  con- 
stitution.    This  naturally  follows  from  the  manifold  factors  of  the 
fluviatile  environment  and  the  directness  with  which  they  impinge 
upon  the  plankton.     Changes  in  volume,   contact  of  shore  and 
bottom,  access  of  heat  and  light,  and  changes  in  chemical  con- 
stituents  are  frequently  both  more   extensive   and  more  widely 
effective  in  the  stream  than  they  are  in  the  other  types  of  aquatic 
environment.     In  consequence,  the  plankton  of  the  stream  is  sub- 
ject to  more  catastrophic  changes  than  that  of  the  lake. 

3.  The  potamoplankton  is  not  characterized  by  any  species 
peculiar  to  it,  nor  by  any  precise  assemblages  of  eulimnetic  organ- 
isms.    It  may  be  distinguished,  in  a  general  way  only,  by  the 
greater  proportion  of  littoral  or  benthal  forms  which  are  mingled 
with  the  more  typical  planktonts. 

Zoological  Laboratory, 

University  of  California, 
May  10,  1904. 


314 


TABLE     I. 

ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Crenothrix, 
Beggiatoa, 
etc.* 

Total 
SchizophycecE 

Clathrocystis 
teruginosa 

Merismopedia 
glauca 

Microcystis 
ichthyoblabe 

Oscillatoria 
spp. 

Total 
Chlorophycece 

Jan  11.. 

399,600,000 

7,477,200 

0 

0 

7,200,000 

277,200 

14,400,200 

"  21.  .. 

105,000,000 

3,046,800 

0 

0 

3,000,000 

46,800 

9,000,200 

"  25  

10,800,000 

14,511,837 

0 

387 

14,400,000 

111,456 

108,000,386 

Feb   3  .  . 

575,000,000 

5,440,000 

0 

0 

5,400,000 

39,600 

9,000,500 

8  

275,400,000 

400 

0 

0 

0 

400 

9,014,800 

"  IS  
"  22.  .. 

602,200,000 
43,200,000 

14,800 
7,200  000 

0 

o 

400 
0 

0 
0 

14,400 
9,477 

3,600,400 
3,159 

Mar   1  . 

0 

0 

o 

0 

0 

0 

400 

8  

79,200,000 

2  700  400 

0 

0 

2  700  000 

400 

3,600  400 

"  15  

40,500,000 

9,000,600 

0 

0 

9,000,000 

600 

22,502,800 

"  22  

21,600,000 

9  ,  000  ,  000 

0 

0 

9,000,000 

0 

600 

"  29.  . 

18,900,000 

6  300  200 

o 

o 

6,300,000 

200 

2,704  200 

Apr   5 

14  400  000 

1  800  100 

o 

o 

1  800  000 

100 

2  700  300 

"  12.  . 

21,600,000 

2  701  100 

o 

0 

2  700  000 

1,100 

1,980  100 

"  19  '. 

21  ,600,000 

18  002,400 

0 

0 

18,000,000 

2,400 

55,800,800 

"  26  

May  3  
"  10 

10,800,000 

42,000,000 
7  200  000 

190,835,200 

174,000,000 
216  057  600 

0 
0 

o 

0 
0 

o 

190,800,000 

168,000,000 
216  000  000 

35,200 

19,200 

57  600 

135,906,400 

212,406,400 
194  531  200 

"  17.  . 

14  400  000 

50  443  200 

o 

o 

50  400  000 

43  200 

64  901  200 

"  24  
"  31  

June  7  

7,200,000 
10,800,000 

18,000,000 

10,800,000 
14,400,000 

21  600  000 

0 
200 

o 

0 
0 

o 

9,000,000 
43,200,000 

14  400  000 

0 
200 

0 

12,602,200 
27,001,600 

21  616  000 

"  14  

18,000,000 

19  800,000 

0 

0 

3  600  000 

0 

46  801  000 

"  21  

43,200,000 

43,200,000 

0 

0 

18  000  000 

0 

21,658  400 

"  28  

25  200  000 

18  000  000 

o 

o 

18  000  000 

0 

34  260  800 

July   5  .  . 

50,400,000 

21,600,040 

0 

0 

21  600  000 

40 

9  049  200 

"  12  

10  800  000 

28  800  060 

o 

o 

28  800  000 

60 

10  851  200 

"  19  

43,200,000 

162  000  400 

400 

o 

162  000  000 

0 

277  340  400 

"  26  

7,200,000 

34  200  000 

o 

o 

34  200  000 

3  200 

31  651  600 

Aug.  2  .... 

21  600  000 

81  003  200 

o 

o 

79  200  000 

0 

45  304  800 

9  

57  600  000 

1  700  600  000 

o 

o 

1  697  000  000 

0 

370  948  400 

"   16  

21,600,000 

212  400  800 

o 

800 

208  800  000 

0 

68  468  800 

"  23  

25,200,000 

100  817  600 

1  600 

2  400 

100  800  000 

13  600 

108  200  000 

"  30.  .  .  . 

14  400  000 

288  121  600 

800 

800 

288  000  000 

27  200 

189  334  400 

Sept.  6  .  . 
"  13.  . 

18,000,000 
28  000  000 

118,800,800 
378  013  500 

800 

o 

0 
0 

111,600,000 
378  000  000 

46,400 
13  500 

87,489,600 
54  042  500 

"  20.  .  .  . 

50  400  000 

72  008  000 

o 

o 

72  000  000 

8  000 

57  684  500 

"  27  

68  400  000 

111  614  400 

o 

o 

108  000  000 

14  400 

70  526  400 

Oct.   4.  . 

34  200  000 

23  400  000 

o 

o 

14  400  000 

10  500 

27  024  000 

"   11.  ... 

21  600  000 

28  803  500 

o 

o 

28  800  000 

3  500 

14  420  000 

"   18  

86  400  000 

3  600  500 

o 

o 

3  600  000 

500 

15  312  500 

"  25  

1,800,000 

52  201  560 

o 

60 

52  200  000 

1  500 

28  833  000 

Nov.  1  .  . 

93  600  000 

21  601  000 

500 

0 

21  600  000 

500 

14  408  000 

8  

176,400  000 

10  800  000 

0 

0 

7  200  000 

0 

3  604  000 

"  15  

190,800,000 

10  400  000 

0 

0 

10  400  000 

0 

34,205  000 

"  22.. 

124  400  000 

7  200  000 

o 

0 

7  200  000 

0 

16  206  000 

"  29  

57,600  000 

7  200'  000 

o 

o 

7  200  000 

0 

7  205  000 

Dec.  6  .  . 

136  800  000 

14  400  000 

o 

o 

14  400  000 

0 

13  500  000 

"  13  

468  000  000 

54  000  000 

o 

o 

54  000  000 

0 

84  600  500 

"  20.  . 

640  800  000 

59  400  000 

o 

o 

59  400  000 

0 

58  500  000 

"  27.. 

497  200 

37  800  000 

o 

o 

37  800  000 

o 

27  000  200 

Average  

55,428  792 

85  909  984 

83 

93 

83  059  615 

15  431 

53  175  104 

315 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Actinastrum 
hantzschii* 

Botryococcus 
braunii 

Ccelastrum 
cambricum* 

Crttcigenia 
rectangularis* 

Golenkinia 
radiata* 

Pediastrum 
boryanum 

Pediastrum 
pertusum 

Jan  11.. 

0 

0 

0 

0 

0 

0 

0 

"  21.. 

0 

0 

0 

3  000  000 

0 

100 

0 

"  25  

0 

0 

0 

0 

0 

0 

387 

Feb   3  .  . 

o 

0 

0 

0 

0" 

0 

300 

8.  ... 

0 

0 

0 

0 

0 

0 

0 

"  IS  

0 

0 

0 

0 

0 

0 

400 

"  22 

0 

0 

0 

0 

o 

0 

0 

Mar   1  ... 

0 

0 

0 

0 

0 

0 

400 

8  
"  IS  
"  22.  . 

.  0 
0 
0 

0 
0 
0 

0 
0 
0 

0 
0 
0 

0 
0 
0 

0 
600 
0 

0 
1,800 
800 

"  29.  . 

0 

0 

o 

0 

0 

600 

200 

Apr.   5  .  . 
"  12.  . 

0 

0 

100 
100 

0 

o 

0 

o 

0 

o 

0 

o 

200 
0 

"   19.  . 

0 

0 

o 

o 

o 

400 

400 

"  26  

May  3  
"  10  
"  17  
"  24  
"  31.  . 

0 

0 
0 
1,800,000 
0 
0 

0 

3,200 
0 
0 
0 

o 

0 

0 
0 
0 
0 

o 

0 

0 
57,600,000 
0 
0 
5  400  000 

1,800,000 

7,200,000 
7,200,000 
0 
0 

o 

3,200 

3,200 
6,400 
4,800 
600 
1  000 

1,600 

0 
4,800 
5,600 
1,600 
600 

June  7 

0 

0 

o 

o 

o 

3  200 

12  800 

"   14.  . 

0 

o 

1  800  000 

o 

o 

32  000 

39  400 

"  21  .. 

0 

•  0 

o 

o 

o 

800 

56  000 

"  28  

0 

o 

3  600  000 

o 

o 

6  400 

55  200 

July   5  .  . 

0 

o 

0 

o 

o 

3  600 

44  800 

"   12.  .. 

1  800  000 

o 

0 

o 

o 

1  200 

49  200 

"   19  
"  26.  . 

10,800,000 
5  400  000 

0 

o 

0 
9  000  000 

61,200,000 
1  800  000 

0 
1  800  000 

4,000 
4  000 

136,000 
247  600 

Aug.  2  .  . 

3  600  000 

o 

0 

9  000  000 

o 

4  800 

295  200 

"   9  . 

5  400  000 

o 

10  800  000 

158  400  000 

o 

2  800 

145  600 

"   16.  . 

5  400  000 

o 

3  600  000 

18  000  000 

o 

1  600 

66  400 

"  23.  . 

10  800  000 

o 

900  000 

14  400  000 

o 

5  600 

194  400 

"  30  

21  600  000 

o 

1  800  000 

21  600  000 

7  200  000 

8  000 

326  400 

Sept  6  . 

10  800  000 

o 

0 

o 

o 

12  000 

177  600 

"   13.  . 

7  200  000 

o 

1  800  000 

7  200  000 

o 

500 

42  000 

"  20... 

7  200  000 

o 

0 

o 

o 

8  500 

76  000 

"  27  

0 

o 

0 

1  800  000 

1  800  000 

65  600 

259  200 

Oct.   4.  . 

1  800  000 

o 

0 

o 

o 

5  500 

18  500 

"   11  

1  800  000 

500 

0 

3  600  000 

o 

8  000 

11  500 

"   18  
"  25.. 

900,000 
5  400  000 

0 

o 

0 

o 

1,800,000 
3  600  000 

0 

o 

3,500 
18  500 

9,000 
14  500 

Nov.  1  .  . 

1,800  000 

o 

o 

0 

o 

5  000 

3  000 

8  

0 

o 

o 

0 

o 

1  000 

3  000 

"  15... 

0 

o 

o 

3  600  000 

o 

3  000 

3  000 

"  22  

0 

o 

o 

o 

o 

4  000 

2  000 

"  29  

0 

o 

o 

0 

o 

500 

0 

Dec.   6.  . 

0 

o 

o 

o 

o 

o 

0 

"  13  

0 

o 

o 

0 

o 

0 

0 

"  20.  . 

0 

o 

o 

o 

o 

o 

o 

"  27  

0 

o 

o 

o 

o 

o 

0 

Average  

199,038 

75 

640  384 

7  153  846 

519  231 

4  sio 

44  372 

316 


TABLE     I  — -Continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Raphidium 

polymorphum*  j 

Scenedesmus 
genuinus* 

Scenedesmus 
obliquus* 

Scenedesmus 
quadricauda* 

Schroederia 
setigera* 

Selenastrum 
bibraianum 

Jan.  1  1  .... 

0 

0 

0 

0 

14,400  000 

0 

"  21  .. 

0 

0 

0 

0 

6  000  000 

o 

"  25  

0 

0 

0 

3  600  000 

7  200  000 

o 

Feb   3 

o 

0 

0 

0 

9  000  000 

o 

8.  .  . 

o 

0 

0 

0 

9  000  000 

o 

"  15  

0 

0 

0 

3,600  000 

43  200  000 

o 

"  22  

0 

0 

0 

o 

0 

0 

Mar   1  .  . 

0 

0 

0 

o 

0 

0 

8  
"  15  
"  22.  . 

900,000 
5,400,000 
16  200,000 

0 
0 
0 

0 
0 

o 

900,000 
0 

o 

1  ,  800  ,  000 
17,100,000 
7  200  000 

0 
0 
0 

"  29  

0 

0 

0 

0 

2,700  000 

o 

Apr   5  .  . 

0 

0 

0 

900  000 

1  800  000 

o 

"   12.  . 

0 

0 

0 

1,800  000 

0 

o 

"   19  
"  26.  . 

7,200,000 
0 

0 
0 

0 
900  000 

1,800,000 
13  500  000 

46,800,000 
108  000  000 

0 

o 

May  3  .  . 

24,000,000 

1,800,000 

0 

23,400,000 

150  000  000 

o 

"  10  
"  17.  . 

7,200,000 
7  200  000 

0 
0 

1,800,000 
0 

70,200,000 
34  200  000 

50,400,000 
21  600  000 

0 

o 

"  24  

3,600,000 

0 

0 

5  400  000 

3  600  000 

o 

"  31  
June  7  .  . 

3,600,000 
9  ,  000  ,  000 

0 
0 

0 
900  000 

3,600,000 
1  800  000 

14,400,000 
9  000  000 

0 

o 

"  14 

21  600  000 

o 

900  000 

0 

21  600  000 

o 

"  21  

"  28.  . 

0 
1  800  000 

0 
0 

0 

o 

7,200,000 
10  800  000 

10,800,000 
10  800  000 

0 

o 

July   5  
"   12  
"   19.  . 

1,800,000 
1,800,000 
75  600  000 

1  ,  800  ,  000 
900,000 
3  600  000 

0 
0 
10  800  000 

1,800,000 
4,500,000 
79  200  000 

3,600,000 
•   1,800,000 
25  200  000 

0 
0 

o 

"  26  
Aug   2  .  . 

5,400,000 
7  200  000 

0 
0 

1,800,000 

o 

900,000 
9  000  000 

1,800,000 
12  600  000 

0 
1  800  000 

9.  . 

57,600  000 

19  800  000 

36  000  000 

39  600  000 

28  800  000 

1  800  000 

"   16  

7,300,000 

7,200,000 

1  800  000 

12  600  000 

7  200  000 

3  600  000 

"  23 

1  800  000 

0 

900  000 

17  100  000 

46  800  000 

900  000 

"  30.  . 

18  000  000 

3  600  000 

2  700  000 

54  000  000 

46  800  000 

7  200  000 

Sept.  6  .  . 

900,000 

0 

8  100  000 

12  600  000 

50  400  000 

3  600  000 

"   13  

0 

0 

3  600  000 

16  200  000 

10  800  000 

1  800  000 

"  20.  . 

7  200  000 

0 

0 

5  400  000 

28  800  000 

0 

"  27.. 

10  800  000 

0 

3  600  000 

12  600  000 

28  800  000 

0 

Oct.   4.  ... 

5,400,000 

900,000 

900,000 

7  200  000 

9  000  000 

1  800  000 

"   11.  . 

0 

0 

0 

5  400  000 

3  600  000 

0 

"   18... 

1  800  000 

0 

0 

4  500  000 

5  400  000 

900  000 

"  25  

0 

0 

1  800  000 

10  800  000 

7  200  000 

0 

Nov.  1  .  . 

0 

0 

0 

1  800  000 

10  800  000 

0 

8.  . 

0 

0 

0 

1  800  000 

0 

0 

"  15  
"  22.  . 

3,600,000 
3  600  000 

0 

o 

1,800,000 
0 

0 
1  800  000 

21,600,000 
10  800  000 

3,600,000 
0 

"  29  

0 

o 

o 

0 

7  200  000 

0 

Dec.  6.. 

0 

o 

o 

0 

12  600  000 

0 

"  13..  . 

0 

o 

o 

900  000 

82  '  800  '  000 

o 

"  20.  . 

0 

900  000 

o 

0 

57  600  000 

o 

"  27  

0 

0 

o 

0 

27  000  000 

0 

Average  .  .  . 

61,230  769 

778  846 

1  505  769 

9  276  923 

21  450  000 

519  235 

317 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN   1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Total 
Bacillariacea; 

Asterionella 
gracillima 

Cyclotella 
kuetzingiana* 

Diatoma 
elongatum 
var.  tenue* 

Fragilaria 
crotonensis 

Fragilaria 
virescens 

Jan   11.. 

239,580 

146,280 

0 

0 

0 

10  000 

"  21  .. 

49,003,100 

1  ,200 

3,000,000 

12,000,000 

0 

o 

"  25 

3  774  901 

10  620 

0 

0 

0 

29  025 

Feb   3  

9,268,530 

5,500 

0 

120,000 

3-rl80 

11  250 

"   8 

7  464  880 

17  200 

0 

60  000 

0 

0 

"   15  
"  22.  . 

29,266,000 
21,911,653 

12,000 
0 

7,200,000 
0 

200,000 
14  400  000 

0 
0 

0 
78  975 

Mar   1 

11  850  400 

0 

0 

3  600  000 

0 

0 

8.  . 

9  080,800 

0 

0 

0 

0 

0 

"  15.. 

24,342,400 

3,200 

8,100,000 

4  500  000 

3,000 

130  000 

"  22.  .  .  . 

42,589,120 

5,920 

16,200,000 

0 

0 

161,600 

"  29 

18  693  300 

17  000 

10  800  000 

0 

0 

72  500 

Apr   5  ... 

8,760,120 

42,320 

900,000 

60  000 

0 

15,600 

"   12 

36  990  300 

170  500 

22  500  000 

0 

19  920 

40  000 

"   19 

794  044  320 

24  059  000 

725  400  000 

0 

19  920 

40  000 

"  26.  . 

3  453,778,080 

891  648  000 

2  880  000  000 

1  800  000 

374  080 

200  000 

May   3 

2  583  832  560 

197  683  200 

891  000  000 

18  000  000 

924  800 

390  000 

"   10.  . 

3  865  257  360 

27  175  680 

2  668  000  000 

10  800  000 

14  469  120 

255  960  000 

"  17.  . 

1  795,608,400 

19  699  200 

1  260  000  000 

14  400  000 

388  800 

4  110  400 

"  24..  .. 

43,487,480 

15,080 

18  000,000 

1  800  000 

0 

1,504  800 

"  31 

138  879  370 

362  880 

88  200  000 

1  800  000 

0 

587  450 

June  7  .  . 

182,162,000 

3  283  200 

55  800  000 

0 

0 

434  000 

"   14 

1  039  619  680 

336  194  880 

46  800  000 

0 

o 

404  000 

"  21  . 

340  702  200 

100  320 

0 

0 

0 

199  800 

"  28.  . 

350  220  000 

34  560 

291  000  000 

1  800  000 

0 

220  000 

July   5 

135  090  000 

3  840 

50  400  000 

1  800  000 

o 

50  000 

"   12.  . 

127  576  000 

0 

72  000  000 

900  000 

0 

120  000 

"   19.  ... 

788,521  600 

o 

561  600  000 

3  600  000 

0 

0 

"  26  
Aug   2  .  . 

87,702,400 
111  750  400 

0 
4  800 

63,000,000 
54  000  000 

0 
0 

0 
0 

0 
0 

9  

443,526,000 

1  200 

401  400  000 

3  600  000 

0 

0 

"   16  
"  23  
'  30.  . 

115,018,656 
180,994,200 
209  793  200 

0 
0 
2  400 

97,200,000 
122,400,000 
93  600  000 

0 
0 
2  700  000 

0 
0 

o 

0 
0 

o 

Sept   6.  . 

186  870  800 

0 

115  200  000 

0 

o 

o 

"   13.  . 

167  208  500 

0 

66  400  000 

0 

o 

6  000 

"  20.  . 

87  481,000 

0 

3  600  000 

0 

o 

0 

"  27  

215,018,800 

0 

57,600,000 

0 

o 

0 

Oct   4.  . 

131  418  900 

0 

37  800  000 

0 

o 

0 

"   11  
"   18  

"  25  

Nov.  1  

46,930,350 
58,436,500 
130,532,250 

54,477,175 

0 
0 
0 

2  000 

7,200,000 
3  ,  600  ,  000 
25,200,000 

14,400  000 

0 
0 
3,600,000 

3  600  000 

0 
0 
0 

o 

75,000 
0 
31,250 

406  125 

8.  ... 

72,584,120 

6,000 

18,000,000 

7  200,000 

o 

609,000 

"  15.. 

132  556  500 

0 

18  000  000 

5  400  000 

o 

1  866  500 

"  22  

295,111,500 

0 

18  000  000 

9  000  000 

o 

1  711  500 

"  29  

218,309,400 

0 

151,200,000 

0 

o 

2  254  000 

Dec.   6  .  . 

308,149,750 

6  OOO 

287  200  000 

900  000 

o 

243  750 

"   13  

864,280,915 

3  240 

811  000  000 

0 

o 

75  625 

"  20.  . 

332  305  000 

10  500 

302  400  000 

900  000 

o 

105  000 

"  27..  . 

239-550  800 

800 

225  000  000 

0 

o 

20  000 

Average  .  . 

396  192  727 

28  860  160 

243  659  615 

2  471  923 

311  593 

5  234  484 

318 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Melosira 
granulata 
var.  spinosa 

Melosira 
granulata 
var.  spinosa* 

Melosira 
•uaricins 

Navicula 
SPP-* 

Surirella 
spiralis 

Synedra 
acus 

Jan  11.. 

0 

0 

0 

0 

0 

14  400 

"  21.  . 

1,000 

24  000  000 

2  200 

3  000  000 

900 

800 

"  25  

9,090 

0 

49,536 

0 

3  870 

42  183 

Feb   3  .  . 

2  204 

0 

3  180 

5  400  000 

300 

5  400 

8  

3,200 

120  000 

6  000 

3  600  000 

880 

1  200 

"  IS  

2,800 

14,400  000 

48  ,  000 

0 

4  000 

400 

•  "  22.  . 

0 

0 

120  042 

0 

6  318 

3  159 

Mar.  1  .  . 

0 

7,200  000 

0 

90,000 

400 

400 

8.  . 

0 

3  600  000 

0 

1  800  000 

800 

400 

"  15.. 

8  640 

8  100  000 

5  200 

900  000 

800 

2  800 

"  22  

60  800 

10  800  000 

800 

1,800  000 

0 

14  000 

"  29  

30,240 

2,700,000 

1,000 

900,000 

400 

8,600 

Apr.   5  

1,620 

900,000 

1,800 

4  500,000 

0 

1  400 

"   12  

3,960 

1,620,000 

0 

4,500,000 

300 

2  100 

"   19.  . 

2  800 

7  200  000 

3  600 

3  600  000 

800 

6  800 

"  26  

595,840 

0 

72  960 

1  800  000 

800 

614  400 

May  3  .  . 

230  400 

9  000  000 

552  960 

6  000  000 

6  200 

2  016  000 

"  10.  ... 

3,421  440 

0 

3  164  160 

21  600  000 

1  600 

9  043  200 

"  17  

259,200 

0 

1,241  200 

64  800  000 

0 

3  801  600 

"  24  
"  31.  . 

109,040 
293,360 

10,800,000 
1  008  000 

126,720 
101  760 

9,000,000 
5  400  000 

200 
40 

86,400 
14  400 

June  7  

26,028,800 

103,320,000 

998,400 

1  800  000 

0 

28  800 

"  14.  . 

0 

128  560  000 

488  320 

3  600  000 

800 

57  600 

"  21.  ... 

32,114,880 

232  200  000 

470  400 

14  400  000 

1  600 

127  200 

"  28  

153,120 

44  100  000 

72  960 

2  700  000 

800 

20  800 

July  5  .  . 

3  628  800 

70  200  000 

34  560 

7  200  000 

1  600 

3  200 

"  12! 

1  ,811  ,520 

41  040  000 

86  400 

2  700  000 

1  600 

3  600 

"   19  
'  26  

Aug.  2  

947,520 
133,920 

316,240 

115,200,000 
20,200,000 

50  400  000 

5,600 
1,000 

12  800 

54,000,000 
3,600,000 

12  600  000 

1,600 
800 

6  400 

1,600 
400 

4  000 

9  
"   16.  . 

1,484,000 
1  250  656 

27,720,000 
0 

800 
6  400 

10,800,000 
7  200  000 

2,000 
1  600 

800 
800 

"  23.. 

366  400 

50  475  000 

12  800 

7  200  000 

3  200 

2  400 

"  30  

5,028,800 

104  490  000 

0 

3  600  000 

o 

6  400 

Sept  6.  . 

1  122  000 

56  250  000 

o 

5  400  000 

o 

800 

"  13.  ... 

1  ,200,000 

64  800  000 

7  000 

16  200  000 

500 

1  500 

"  20  

2,227,000 

33  480  000 

30  000 

1  800  000 

500 

5  000 

"  27.  . 

5  499  840 

146  520  000 

94  080 

9  000  000 

4  800 

17  600 

Oct.   4.  . 

805  800 

40  500  000 

18  900 

9  900  000 

o 

2  000 

"   11  

840,000 

37  800  000 

55  350 

0 

o 

3  500 

"   18  

436,650 

27  360  000 

348  000 

12  600  000 

0 

8  000 

"  25... 

736  000 

56  700  000 

214  500 

5  400  000 

1  000 

2  000 

Nov.  1  .  . 

83,200 

3  600  000 

70  550 

12  600  000 

o 

12  500 

8.  .  . 

98  700 

10  800  000 

25  120 

19  800  000 

o 

19  000 

"  15  

7  000 

22  680  000 

57  400 

21  600  000 

3  000 

6  000 

"  22  

60,000 

194  400  000 

0 

23  400  000 

o 

4  000 

"  29  

2,000 

13  500  000 

9  500 

18  000  000 

o 

3  500 

Dec.  6  .... 

0 

5  400  000 

0 

6  300  000 

o 

1  500 

"  13  

0 

0 

0 

6  300  000 

o 

2  600 

"  20.  . 

o 

4  500  000 

0 

4  500  000 

o 

4  400 

"  27  

o 

o 

o 

2  700  000 

o 

1  600 

Average  .  .  . 

1  181  125 

34  762  365 

148  626 

8  569  038 

1  612 

308  330 

319 


TABLE     I- — continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

It 

S  s 

11 

(/) 

Total 
Conjugates 

Closterium 
acerosum 

Closterium 
gracile 

Closterium 
lunula 

Staurastrum 
gracile 

Total 
Protozoa 

Tan  11.. 

40  ,  000 

0 

0 

0 

0 

0 

123,518,320 

"  21  

0 

80 

0 

0 

80 

0 

36,316,000 

"  25  

0 

0 

0 

0 

0 

0 

43,464,482 

Feb   3  .  . 

0 

100 

0 

0 

100 

0 

21,691,300 

8  

60,000 

80 

0 

0 

80 

0 

6,096,160 

"   15 

3,600  000 

400 

0 

0 

400 

0 

19,093,280 

"  22.  . 

0 

0 

0 

0 

0 

0 

44,060,478 

Mar   1 

900  000 

0 

0 

0 

0 

0 

11,727,360 

8 

3,600  000 

40 

0 

0 

40 

0 

2,516,240 

"   15.. 

2,480,000 

1,000 

400 

0 

600 

200 

22,368,600 

"  22  

13,620,000 

600 

200 

0 

400 

0 

29,817,200 

"  29  

4,200,000 

1,000 

0 

400 

600 

0 

7,169,620 

Apr   5  .  . 

2,340,000 

440 

40 

100 

200 

0 

15,052,540 

"   12  

4,500,000 

300 

200 

100 

0 

0 

29,011,320 

"  19  

"  26.  . 

23,580,000 
82  800  000 

1,200 
3,200 

0 
0 

800 
0 

400 
3,200 

0 
0 

39,856,000 
94,337,920 

May   3  .  . 

240,000,000 

3,200 

3,200 

0 

0 

0 

1,081,381,200 

"   10  
"   17.  . 

813,600,000 
367  200,000 

1,800,000 
3,000 

0 
800 

0 
1  ,600 

0 
800 

0 
0 

222,233,400 
252,834,800 

"  24..  .. 

1,800,000 

7,200 

1,000 

200 

6,000 

0 

121,175,320 

"  31  

June  7  ... 

37,800,000 
17,100,000 

62,200 
1,200 

400 
800 

0 
0 

1,800 
400 

0 
0 

31,584,920 
27,679,000 

"   14 

21  600  000 

2  000 

200 

0 

1,800 

0 

49,614,800 

"  21 

79  200  000 

1  100 

800 

0 

300 

0 

230,167,200 

"  28.  . 

5,400,000 

800 

0 

0 

0 

0 

191,626,440 

Tulv   5 

1  800  000 

800 

0 

0 

0 

400 

78,477,400 

"  12 

5  400  000 

0 

0 

0 

0 

0 

49,852,520 

"   19.  . 

39,600,000 

1,200 

400 

800 

0 

0 

295,478,560 

"  26  

900,000 

60 

60 

0 

0 

0 

121,362,600 

Aug   2  . 

0 

40 

40 

0 

0 

0 

112,224,400 

9.  . 

0 

80 

80 

0 

0 

0 

566,013,480 

"   16  

5,400,000 

120 

60 

0 

60 

0 

166,746,460 

"  23 

0 

0 

0 

0 

0 

0 

129,617,660 

"  30.  . 

6,300  000 

240  200 

120 

0 

80 

0 

95,553,600 

Sept.  6  

3,600,000 

2,400 

2,400 

0 

0 

0 

137,009,680 

"   13  
"  20.  . 

7,200,000 
16,200,000 

1,060 
120,620 

500 
500 

500 
2,500 

60 
120 

0 
0 

50,995,120 
65,106,000 

"  27.  .. 

1,800,000 

6,800 

200 

6,400 

200 

0 

46,830,100 

Oct   4 

42,300  000 

241  000 

0 

1,000 

500 

500 

49,825,580 

"  11  .. 

1,800,000 

1,160 

80 

1,000 

80 

0 

15,982,080 

"   18.  ... 

13,500,000 

160 

80 

0 

80 

0 

19,122,540 

"  25 

27  000  000 

500 

500 

0 

0 

0 

6,776,060 

Nov   1  ... 

5,400,000 

9,500 

2,500 

500 

6,500 

500 

26,343,120 

8 

16  200  000 

2  000 

1  000 

0 

1,000 

0 

15,566,060 

"  15 

37  800  000 

1  100 

1,000 

0 

1,000 

0 

36,542,100 

"  22.  . 

23  400  000 

200 

0 

0 

2,000 

0 

24,435,040 

"  29  

30,600,000 

0 

500 

0 

20 

0 

74,444,400 

Dec   6.  . 

8  100  000 

520 

0 

0 

0 

0 

57,242,080 

"   13  

27,000,000 

0 

0 

0 

0 

0 

149,284,900 

"  20.  . 

5  400  000 

0 

0 

o 

0 

0 

116  833,160 

"  27.  . 

10  800  000 

200 

200 

0 

0 

0 

68,456,620 

Average  .  . 

39  639  231 

48  456 

348 

305 

556 

31 

102  220  941 

TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Total 
Mastigophora 

Bicosceca 
lacustris 

Chilomonas 
paramcecium 

Dinobryon 
sertularia 

Dinobryon 
sertularia 
var. 
angulatum 

Dinobryon 
sertularia 
var. 
divergens 

Jan.  11.. 

122,484,100 

0 

0 

0 

0 

0 

"  21  

36,086,000 

0 

0 

0 

0 

0 

"  25  

43,208,127 

0 

0 

0 

0 

0 

Feb   3  .  . 

20,321,700 

0 

0 

0 

0 

0 

8  

5,461  ,600 

0 

0 

0 

0 

0 

"  15.  . 

18  039  600 

0 

0 

0 

0 

o 

"  22.  . 

43,400  000 

0 

0 

0 

0 

o 

Mar.  1  .  . 

9,940,000 

0 

0 

0 

0 

0 

8.  . 

2  009  200 

0 

0 

0 

0 

o 

"  is.:.... 

"  22  

22,035,600 
29,539,000 

0 
0 

0 
0 

0 
17  800 

0 
0 

0 
0 

"  29.  . 

6  864  800 

0 

0 

0 

0 

o 

Apr.   5  .  . 
"  12.  . 

14,809,800 
27  662  900 

0 
0 

0 

o 

0 

o 

0 

o 

0 

o 

"   19.  . 

38,507  900 

0 

60  000 

8  000 

35  040 

8  000 

"  26  

86,614  400 

0 

10  800  000 

1  806  400 

598,400 

1  555  200 

May  3  .  . 

1  063  924  800 

o 

7  200  000 

2  764  800 

0 

2  104  100 

"   10  

203,922,800 

o 

1  800  000 

16  153  600 

4,432  000 

39  648  000 

"  17  

"  24.. 

231,154,200 
120  175  000 

0 

o 

0 
1  800  000 

43,200 
3  600 

0 
0 

1,584,000 
18  000 

"  31  

29,293,200 

o 

o 

o 

o 

o 

June  7  .  . 

19  855  400 

460  800 

o 

o 

14  400 

56  000 

"  14..'... 

43  112  400 

3  801  600 

o 

o 

0 

16  000 

"  21  

218,131,200 

432  000 

o 

3  200 

12  000 

o 

"  28  

185,098,240 

86  400 

o 

o 

172  800 

47  040 

July   5  .  . 

42,053  200 

72  000 

o 

o 

0 

o 

"  12  

45,923  600 

14*400 

o 

o 

0 

o 

"   19 

294  724  520 

o 

o 

o 

o 

o 

"  26.  . 

120  850  000 

o 

o 

o 

o 

o 

Aug.  2  .  . 

107,710  800 

0 

o 

o 

0 

o 

9  

496,927,200 

0 

o 

o 

0 

o 

"  16.  . 

166  452  800 

o 

o 

o 

o 

o 

"  23  

128  830  460 

o 

o 

o 

o 

o 

"  30  

95,423  200 

0 

o 

o 

>          0 

o 

Sept  6  .  . 

76  982  440 

o 

o 

o 

o 

o 

"  13  

49  515  000 

7  500 

o 

o 

o 

o 

"  20  

63,144  000 

o 

o 

o 

o 

o 

"  27.  . 

45  854  000 

218  400 

o 

o 

o 

o 

Oct.   4.  . 

48  193  000 

251  000 

1  800  000 

o 

o 

o 

"  11  

15,129  540 

486  '  000 

o 

o 

o 

o 

"  18.  . 

17  367  000 

25  000 

o 

o 

o 

Q 

"  25  

5  416  500 

13  500 

o 

o 

o 

Q 

Nov.  1  .  . 

25,325  500 

2  000 

o 

o 

o 

o 

"   8.  . 

14  564  000 

o 

3  600  000 

25  000 

o 

Q 

"  15  

36  Oil  000 

o 

o 

o 

o 

Q 

"  22  
"  29  

23,494,000 
73,719  000 

0 

0 

0 

o 

0 
38  500 

0 

o 

0 

o 

Dec.  6  .  . 

56  400  500 

o 

o 

o 

o 

o 

"  13  

148,740  000 

o 

1  800  000 

o 

o 

o 

"  20  

116,344  800 

o 

o 

247  200 

6  000 

o 

"  27  

67  965  800 

o 

o 

69  600 

o 

o 

Average  

95,852  602 

112  896 

555  000 

407  602 

101  358 

866  083 

321 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Dinobryon 
sertularia 
var. 
stipitatum 

Eudorina 
elegans 

Euglena 
acus 

II 
^ 

Euglena 
oxyuris 

Euglena 
oxyuris* 

Jan     11.. 

0 

0 

0 

0 

0 

0 

"     21 

0 

0 

0 

0 

100 

0 

"     25.  . 

0 

0 

0 

0 

0 

0 

Feb.     3  

0 

0 

0 

0 

0 

0 

"       8 

0 

0 

0 

0 

0 

0 

"     15 

0 

0 

0 

0 

0 

0 

"     22.  . 

0 

0 

0 

0 

0 

0 

Mar       1 

0 

0 

0 

0 

0 

0 

8.  . 

0 

0 

0 

0 

0 

0 

"     15.  . 

0 

3,600 

0 

40,000 

0 

40,000 

"     22.  . 

0 

800 

0 

0 

0 

0 

"     29  
Apr       5  .  . 

0 
0 

2,600 
2,800 

0 
0 

0 
0 

0 
0 

0 
0 

"     12  

0 

1,800 

100 

0 

0 

0 

"      19  

9,960 

36,000 

0 

0 

100 

0 

"     26 

1,830,400 

240,000 

800 

0 

0 

0 

May      3  .  . 

4,883,200 

240,000 

0 

0 

3,200 

0 

"      10  

24,608,000 

48,800 

0 

0 

0 

0 

"     17 

28,800 

32,800 

0 

90,000 

0 

180,000 

"     24.  . 

0 

1,000 

0 

0 

0 

0 

"     31  .. 

0 

400 

0 

0 

0 

0 

June     7 

0 

9,600 

0 

0 

0 

0 

"      14.  . 

0 

60,000 

0 

900  ,  000 

1,600 

0 

"     21  .. 

0 

30,400 

0 

0 

2,400 

0 

"     28  

0 

4,000 

0 

0 

0 

1,800,000 

July      5  .  . 

0 

400 

400 

0 

800 

0 

"      12  

0 

800 

400 

0 

1,200 

0 

"      19 

o 

7,600 

0 

0 

400 

3,600,000 

"     26.  . 

0 

4,000 

0 

0 

2,400 

3,600,000 

Aug.     2  ... 

0 

8,000 

800 

120,000 

3,200 

1,800,000 

9  

0 

400 

800 

0 

1,200 

3,600,000 

"      16 

0 

800 

800 

120,000 

0 

3,600,000 

"     23.  . 

0 

3,200 

1,600 

0 

6,400 

120,000 

"     30.  .     . 

0 

2,400 

800 

0 

3,200 

4,500,000 

Sept     6 

0 

40 

1,600 

900,000 

10,400 

5,400,000 

"      13.  . 

0 

500 

0 

0 

1,500 

3,600,000 

"     20.  ... 

0 

2,000 

1,500 

0 

1,000 

1,800,000 

"     27  

0 

1,600 

6,400 

1,800,000 

9,600 

9,000,000 

Oct.      4.  . 

0 

0 

1,500 

3,600,000 

1,000 

2,700,000 

"      11  

0 

0 

1,000 

1,800,000 

500 

1,800,000 

"      18.  ... 

0 

0 

0 

0 

0 

900,000 

"     25.  . 

0 

0 

0 

0 

0 

0 

Nov.  '  1  .  . 

0 

0 

0 

0 

0 

'                          0 

"     '  8 

0 

0 

0 

1,800,000 

•      0 

0 

"     15  
'     22.  .. 

0 
0 

0 
0 

1,000 
0 

0 
0 

0 
0 

1,800,000 
0 

"     29  

0 

0 

0 

0 

0 

120,000 

Dec.      6  .  . 

0 

o 

0 

0 

0 

0 

"     13  

0 

500 

0 

0 

0 

0 

"     20.  ... 

22,000 

0 

0 

0 

0 

0 

"     27.. 

0 

0 

0 

0 

0 

0 

603   911 

14  362 

375 

214,807 

963 

960,769 

322 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

O.eg 

J>2 

"3>-« 

^B 

Euglena 
viridis* 

Glenodinium 
cinctum 

Glenodinium 
cinctum* 

Gonium 
pectorale 

Lepocinclis 
ovum 

Lepocinclis 
ovum* 

Tan.  1  1  .... 

0 

0 

0 

80  000 

0 

0 

o 

"  21.  . 

0 

0 

0 

o 

o 

100 

Q 

"  25.  .. 

0 

0 

0 

0 

o 

o 

Q 

Feb   3 

0 

0 

0 

o 

o 

o 

Q 

8.  . 

0 

0 

0 

0 

o 

o 

Q 

"  15.. 

0 

0 

0 

0 

0 

o 

Q 

"  22  

0 

0 

0 

0 

0 

o 

o 

Mar   1  .  . 

0 

0 

0 

0 

o 

o 

Q 

"   8  
"  15  
"  22.  . 

0 
200 
400 

0 
0 
0 

400 
200 
0 

200,000 
240,000 
4  260  000 

0 
0 

o 

0 
0 

o 

0 
0 

Q 

"  29  

0 

0 

0 

240  000 

0 

o 

Apr   5  .  . 

0 

0 

0 

240  000 

o 

200 

o 

"  12.  .. 

0 

0 

0 

120  000 

0 

200 

Q 

"  19  

400 

0 

1,200 

240  000 

o 

800 

Q 

"  26  
May  3  

3,200 
0 

360,000 
120,000 

0 
0 

0 
0 

0 
22  400 

0 

o 

0 

Q 

"  10.  
"  17.  . 

0 
0 

120,000 
3,600,000 

0 

o 

0 
90  000 

200 
800 

0 

o 

0 
Q 

"  24.  . 

0 

630,000 

o 

o 

o 

200 

1  800  000 

"  31  

0 

60,000 

0 

o 

o 

400 

Q 

June  7  ... 

0 

0 

o 

900  000 

o 

o 

180  000 

"   14  

1,600 

2,700,000 

o 

60  000 

0 

800 

420  000 

"  21  
"  28.  . 

3,200 
0 

7,200,000 
900  000 

0 

o 

7,200,000 

o 

0 

o 

2,400 
5  600 

240,000 

Q 

July   5  .  . 

0 

120,000 

o 

0 

o 

1  600 

Q 

"  12.. 

0 

2  700  000 

o 

o 

o 

800 

"   19.  . 

2  400 

3  600  000 

o 

7  200  000 

o 

4  400 

"  26.  ..  . 

3,200 

14  400  000 

o 

2  700  000 

o 

30  000 

900  000 

Aug   2  .  . 

1  600 

7  200  000 

20  000 

12  600  000 

o 

50  400 

9.  . 

4  800 

7  200  000 

400 

25  200  000 

o 

6  400 

"   16  

0 

5  400  000 

o 

5  400  000 

o 

800 

720  000 

"  23 

4  800 

4  500  000 

o 

o 

o 

14  400 

"  30.  . 

8  000 

2  700  000 

800 

*    900  000 

800 

43  200 

Sept.  6  .  . 

800 

3  600  000 

0 

900  000 

o 

11  200 

240  000 

"   13  
"  20.  . 

1,000 
3  000 

1,800,000 
0 

0 
500 

0 

120  000 

0 

o 

1,000 

5  000 

0 

"  27.  . 

6  400 

1  800  000 

0 

o 

3  200 

8  000 

Oct.   4  
"   11  .. 

0 
0 

6,300,000 
120  000 

0 

o 

0 

o 

0 

o 

2,500 
2  000 

1,800,000 

Q 

"   18  

0 

0 

o 

o 

o 

500 

120  000 

"  25..'  

0 

0 

o 

o 

o 

o 

Q 

Nov.  1  .  . 

0 

o 

o 

o 

o 

500 

o 

8  

0 

o 

o 

o 

o 

o 

Q 

"  15  

0 

o 

o 

o 

o 

o 

Q 

"  22.  . 

o 

o 

o 

o 

o 

O 

Q 

"  29  

o 

o 

o 

o 

o 

Q 

120  000 

Dec.   6.  . 

0 

o 

o 

o 

0 

Q 

"   13.  ... 

o 

1  020  000 

o 

60  000 

o 

Q 

Q 

"  20  

o 

o 

o 

900  000 

o 

o 

o 

"  27.  . 

o 

o 

o 

960  000 

Q 

Q 

Average  

8  653 

1  571  731 

452 

1  360  192 

526 

3  719 

401  538 

323 


TABLE     I  —  continued.    • 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Mallomonas 
producta 

Pandorina 
morum 

Peridinium 
tabulatum 

Peridinium 
tabulatum* 

Phacus 
longicauda 

Phacus 
pleuronectes 

Platydorina 
caudata 

Pleodorina 
calif  arnica 

Jan.  11  
"  21.  . 

0 
0 

0 
0 

400 
100 

0 
0 

0 
100 

0 
0 

0 
0 

0 
0 

"  25  

0 

0 

0 

0 

0 

0 

0 

Feb   3  .  . 

0 

0 

0 

0 

0 

0 

100 

0 

8  

0 

0 

400 

0 

0 

0 

0 

0 

"  15 

0 

0 

0 

0 

0 

0 

0 

0 

"  22. 

0 

0 

0 

0 

0 

0 

0 

0 

Mar   1  .  . 

0 

0 

400 

0 

0 

0 

0 

0 

8 

0 

0 

0 

0 

0 

0 

0 

0 

"   IS.  . 

0 

0 

600 

0 

0 

0 

0 

0 

"  22.  . 

0 

0 

0 

0 

0 

0 

0 

0 

"  29.  . 

0 

0 

200 

0 

600 

0 

0 

0 

Apr   S  .  . 

0 

0 

200 

0 

0 

0 

0 

0 

"   12.  . 

0 

0 

500 

0 

600 

0 

0 

0 

"   19  

0 

800 

400 

0 

1,600 

0 

0 

0 

"  26. 

0 

48  ,  000 

0 

0 

3,200 

0 

0 

0 

May   3  .  . 

12,800 

48  ,  400 

0 

0 

3,200 

0 

0 

0 

"   10 

0 

0 

0 

0 

0 

0 

0 

0 

"   17.  . 

0 

800 

0 

0 

0 

0 

0 

0 

"  24.  . 

0 

0 

0 

0 

0 

0 

0 

0 

"  31  

0 

0 

0 

0 

400 

0 

0 

0 

June  7  .  . 

835,200 

8,000 

0 

120,000 

200 

0 

0 

0 

"   14 

28  800 

60,000 

0 

900,000 

8,800 

800 

0 

0 

"  21  
"  28.  . 

28,800 
28,800 

40,800 
9,600 

2,400 
8,800 

1,200,000 
79,200,000 

8,800 
•  4,800 

0 
800 

0 
0 

0 
0 

July   5  .  . 

0 

400 

2,000 

5,400,000 

4,800 

0 

0 

0 

"  12. 

0 

800 

18,800 

10,800,000 

3,200 

400 

400 

0 

"   19.  . 

0 

12,000 

49  ,  600 

86,400,000 

3,200 

400 

400 

120 

"  26.  . 

0 

63,200 

66,800 

15,300,000 

6,800 

800 

0 

400 

Aug.  2  
9.  . 

800 
0 

59,200 
1,200 

12,000 
7,200 

120,000 
120,000 

11,200 
4,800 

2,000 
0 

0 
0 

0 
0 

"   16.  .. 

0 

0 

3,200 

0 

8,000 

0 

0 

0 

"  23  

0 

2,400 

6,400 

1,800,000 

4,800 

800 

.  0 

60 

"  30.  . 

0 

3,200 

6,400 

0 

8,000 

1,600 

0 

0 

Sept.  6.  .  .  . 

0 

2,400 

0 

0 

12,800 

1,600 

0 

0 

"   13 

3  000 

0 

0 

0 

3,000 

1,000 

0 

0 

"  20.  . 

01 

0 

1,500 

0 

7,000 

500 

0 

0 

"  27.. 

1,600 

100 

4,800 

0 

35,200 

4,800 

0 

0 

Oct   4.  . 

„ 

0 

0 

0 

7,000 

0 

0 

0 

"   11  
"   18 

S 

0 
500 

0 
0 

0 
0 

1,500 
1  ,000 

0 
0 

0 
0 

0 
0 

"  25 

0 

0 

0 

0 

500 

0 

0 

0 

Nov.  1  .  . 

0 

0 

0 

0 

500 

0 

0 

0 

8  
"  15.  . 

0 
0 

0 
0 

0 
0 

0 
0 

1,000 
1,000 

0 
0 

0 
0 

0 
0 

"  22.  . 

0 

0 

0 

0 

0 

0 

0 

0 

"  29.  ... 

0 

0 

0 

0 

0 

0 

0 

0 

Dec.   6  .  . 

0 

0 

0 

0 

0 

0 

0 

0 

"   13  

0 

0 

0 

180,000 

0 

0 

0 

0 

"  20.  . 

o 

0 

0 

0 

0 

0 

0 

0 

"  27.  .. 

0 

0 

0 

0 

0 

0 

0 

0 

Average.  . 

17  520 

6  957 

3  711 

3  875  769 

3,031 

298 

17 

11 

(22) 


324 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Syncrypta 
volvox 

^ 

S  to 
?,5 
<0 

Synura 
uvella* 

Trachelomonas 
acuminata 

Trachelomonas 
acuminata* 

Trachelomonas 
hispida 

Trachelomonas 
volvocina* 

Jan.  11  
"  21.  . 

0 
0 

100 
5,600 

0 
0 

0 
0 

0 
0 

3,600 

3,800 

0 
0 

"  25.  . 

0 

7,740 

0 

0 

0 

387 

0 

Feb   3  . 

0 

1,600 

0 

0 

0 

4,600 

480,000 

8.  . 

0 

800 

0 

0 

0 

800 

60,000 

"  15  

0 

10,800 

0 

0 

3,600,000 

28,800 

0 

"  22  
Mar   1  .  . 

0 
800 

0 
8,800 

0 
0 

0 
0 

0 
0 

0 

800 

200,000 
900,000 

8  
"   15.  . 

400 
1,200 

8,800 
109,200 

0 
0 

0 
0 

0 
0 

800 
0 

900,000 
1,800,000 

"  22.  .. 

0 

221  ,600 

60,000 

0 

0 

0 

10,800,000 

"  29  
Apr   5  .  . 

0 
0 

320,600 
166,600 

0 
0 

0 
200 

0 
0 

200 
0 

900,000 
1,800,000 

"   12  

100 

17,800 

0 

0 

0 

0 

0 

"   19 

0 

126,000 

60,000 

0 

0 

0 

9,000  000 

"  26  . 

0 

121  ,600 

120,000 

0 

0 

0 

4,500,000 

0 

102  400 

0 

0 

0 

3  200 

3  600  000 

"   10.  . 

0 

38,400 

0 

0 

0 

0 

9,000  000 

"   17.  . 

0 

21  ,600 

0 

0 

3,600,000 

0 

14,400,000 

"  24  
"  31  . 

0 
0 

1,400 
200 

0 
0 

0 
0 

0 
60,000 

200 
0 

360,000 
180,000 

June  7  

0 

0 

0 

0 

0 

0 

4,500,000 

"   14  
"  21  .. 

0 
0 

0 
1,600 

0 
0 

0 
800 

120,000 
7,200,000 

0 
0 

7,200,000 
147  600  000 

"  28.  . 

0 

800 

0 

0 

6,300,000 

0 

•  38  700,000 

Tuly  5  . 

0 

0 

0 

400 

1  800  000 

0 

1  800  000 

"  12!  ! 

0 

1  ,200 

0 

800 

900,000 

0 

10  800  000 

"   19  

0 

0 

0 

800 

3,600,000 

0 

86,400,000 

"  26 

0 

0 

0 

2  000 

3  600  000 

9  200 

42  300  000 

Auff   2  .  . 

0 

0 

0 

12,800 

600,000 

800 

18  000  000 

.?•     g 

0 

0 

0 

800 

3,600,000 

0 

252,000,000 

"  16.. 

0 

0 

0 

4,000 

3,600,000 

1  ,600 

93,600,000 

"  23.  . 

0 

0 

0 

3  200 

1  800  000 

800 

65  700  000 

"  30.  . 

0 

0 

0 

8  800 

1  800  000 

1  ,600 

18  000  000 

Sept.  6  
"   13.. 

0 
0 

0 
0 

0 
0 

4,000 
0 

5  ,  400  ,  000 
0 

800 
1  000 

16,200,000 
6  300  000 

"  20.  . 

0 

4,000 

0 

1  500 

0 

0 

1  800,000 

"  27  
Oct.   4.  . 

0 
0 

1,600 
0 

0 
0 

4,800 
500 

3,600,000 
1  800  000 

1,600 
0 

9,000,000 
11  700  000 

"  11  

0 

0 

0 

0 

120  000 

0 

1  ,800  000 

"   18  
"  25  

Nov.  1  .  . 

0 
0 

0 

0 
500 

2,000 

0 
0 

0 

0 
0 

0 

900,000 
0 

120,000 

0 
0 

0 

2,700,000 
5,400,000 

1,800,000 

8  . 

1  000 

16  000 

0 

0 

0 

0 

1  800  000 

"  15.. 

0 

9  000 

0 

0 

0 

0 

0 

"  22  

0 

94,000 

0 

0 

1  800  000 

0 

5  400,000 

"  29  

4,500 

1  ,999,500 

1  ,320,000 

0 

0 

0 

3,600,000 

Dec.   6  .  . 

13,500 

1  693  500 

2  280  000 

0 

0 

0 

900,000 

"  13  

"  20.  . 

2,000 
6  200 

78,000 
2  764  800 

2,760,000 
900  000 

0 
0 

0 

o 

500 

o 

2,400,000 
0 

"  27...  . 

800 

395  200 

300  000 

0 

0 

o 

2  700  000 

Average.  . 

625 

179.138 

150.000 

873 

t  .094.615 

1.251 

17.672.692 

325 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Total 
Rhizopoda 

Arcella 
discoides 

A  rcella 
vulgaris 

Centropyxis 
aculeata 

Centropyxis 
aculeata  var. 
ecornis 

Cochliopodium 
bilimbosum 

Cyphoderia 
margaritacea 

Difflugia 
acummata 

Jan  11.. 

440,500 

100 

0 

0 

0 

0 

0 

0 

"  21  

32,800 

100 

200 

0 

0 

100 

0 

0 

"  25 

66  338 

387 

387 

387 

1,161 

20,898 

774 

1,935 

Feb   3  .  . 

122,900 

0 

0 

0 

0 

1,300 

0 

0 

8 

4  880 

0 

0 

0 

0 

3,200 

0 

0 

"   15  . 

34  880 

800 

0 

800 

800 

0 

0 

80 

"  22.  . 

141  ,524 

632 

25,272 

12,636 

9,477 

3,159 

0 

0 

Mar   1 

11  200 

400 

0 

400 

400 

400 

0 

0 

8.  . 

11  ,720 

400 

0 

400 

1,200 

0 

400 

40 

"   15.  . 

7,600 

600 

0 

0 

400 

0 

0 

0 

"  22  
"  29 

4,800 
61  400 

400 
400 

0 
0 

0 
200 

0 
0 

0 
0 

0 
0 

0 
0 

Apr   5  .  .  . 

700 

100 

100 

100 

0 

0 

0 

0 

"   12 

3  520 

300 

200 

0 

20 

100 

0 

0 

"   19  
"  26  . 

7,300 
6,720 

400 
0 

0 

0 

0 

0 

0 
0 

400 
0 

400 
0 

0 
0 

May   3  .  . 

26,000 

0 

0 

0 

400 

0 

0 

0 

"   10  
"   17  
"  24.  . 

49,800 
23,800 
9,320 

0 
2,400 
600 

0 
0 
0 

0 
0 
0 

1,600 
800 
200 

0 
0 
0 

0 
0 
400 

0 
0 
80 

"  31  
June  7  .  . 

8,920 
23  600 

400 
800 

0 
200 

200 
0 

0 
0 

0 
0 

400 
0 

200 
3,200 

"   14.  . 

21,600 

1,600 

800 

0 

0 

0 

0 

0 

"  21 

21  600 

1  600 

800 

0 

0 

0 

800 

0 

"  28.  . 

37  000 

800 

0 

0 

0 

0 

800 

100 

July   5 

19  360 

0 

0 

1  200 

400 

0 

400 

400 

"   12 

26  000 

800 

0 

800 

200 

o 

1  600 

1  200 

"   19.  . 

28  800 

0 

800 

400 

400 

0 

400 

0 

"  26.  . 

4  800 

400 

400 

0 

0 

0 

400 

0 

Aug   2 

16  800 

800 

4  800 

0 

0 

1  600 

0 

0 

9.  . 

7  280 

0 

1  600 

0 

400 

1,600 

40 

40 

"   16.  . 

24  060 

0 

2  400 

800 

0 

800 

0 

800 

"  23 

36  800 

800 

5  600 

0 

0 

800 

0 

0 

"  30  
Sept  6  ...  . 

23,200 

0 

20  800 

800 
800 

5,600 
800 

0 
800 

0 
0 

1,600 
3,200 

0 
0 

0 
1  600 

"  13  

28,000 

500 

500 

0 

0 

6,000 

0 

1  ,000 

"  20  
"  27.  . 

19,000 
59  200 

500 
1  600 

500 
1  600 

500 
0 

500 
0 

1,000 
8,000 

1,500 

o 

500 
3  200 

Oct.   4.  . 

912,580 

0 

40 

0 

40 

500 

500 

0 

"  11 

9  000 

0 

1  000 

0 

0 

0 

0 

0 

"   18.  . 

10  000 

0 

0 

0 

500 

2  000 

1  000 

0 

"  25..  . 

25,060 

1,000 

1,500 

1,000 

1,000 

500 

500 

500 

Nov   1. 

32  060 

500 

1  000 

1  000 

2  000 

500 

0 

500 

8.  . 

37  060 

1  000 

1,000 

1  ,000 

1,000 

0 

0 

1  000 

"   15  

42,000 

1,000 

0 

5,000 

4,000 

0 

0 

0 

"  22 

190  400 

0 

0 

2  000 

4  000 

6  000 

o 

0 

"  29.  . 

3  400 

o 

0 

0 

500 

500 

o 

0 

Dec   6. 

121  000 

o 

0 

0 

0 

1  000 

o 

0 

"   13.  . 

600 

0 

0 

0 

0 

600 

0 

0 

"  20  

1  040 

40 

0 

0 

0 

1  ,000 

0 

0 

"  27.  . 

220 

0 

0 

0 

0 

0 

o 

0 

55  364 

465 

1  098 

570 

604 

1  284 

198 

315 

326 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Difflugia 
globulosa 

Difflugia 
lobostoma 

Difflugia 
pyrijormis 

Total 
Heliozoa 

Nuclearia 
delicatula 

a 
ll 

SG 

EH 

Amphileptus 
•spp. 

Carchesium 
lachmanni 

Jan   11.. 

100 

100 

0 

0 

0 

593,420 

0 

26  500 

"  21  

400 

200 

0 

0 

0 

197,100 

0 

37  000 

"  25.  . 

9,675 

7  353 

0 

0 

0 

190  017 

13  545 

45  666 

Feb.   3  .  . 

500 

100 

0 

0 

0 

1,246,300 

1  600 

54  700 

8  
"   15.  . 

800 
9,200 

80 
2,000 

0 
0 

0 
0 

0 
0 

629,680 
1  016  000 

800 
4  400 

197,600 
164  800 

"  22.  . 

6,318 

0 

632 

0 

0 

518,954 

0 

50  544 

Mar   1  .  . 

4,000 

800 

400 

0 

0 

1  773  360 

400 

46  400 

8.  . 

2,800 

800 

40 

0 

0 

492  920 

800 

54  800 

"   15.  .. 

2,600 

1,400 

0 

200 

0 

324,200 

200 

89  600 

"  22 

1  600 

800 

0 

2  000 

0 

267  800 

400 

22  000 

"  29.  . 

200 

0 

0 

400 

0 

241  420 

400 

10  200 

Apr   5 

100 

0 

0 

500 

0 

241  440 

o 

3  100 

"   12  
"   19.  . 

1,000 
1,600 

800 
1,200 

0 
100 

100 
0 

0 
0 

1,342,500 
1,340  800 

300 
0 

2,400 
13  200 

"  26.  ... 

3,200 

0 

0 

3,200 

0 

7,710  400 

0 

99*200 

May  3  .  . 

22,400 

3  200 

0 

0 

0 

17  404  800 

0 

83  200 

"  10  

30,400 

0 

3,200 

0 

0 

18  260  800 

0 

6  400 

"  17  

800 

800 

0 

0 

0 

21,654,400 

1  600 

0 

"  24.  . 

3  640 

200 

200 

0 

0 

990  800 

o 

200 

"  31.  . 

3,840 

400 

0 

0 

0 

2  282  400 

o 

600 

June  7  
"   14.  . 

9,600 
5  600 

8,000 
1  600 

200 
800 

0 
0 

0 

o 

7  ,  800  ,  000 
6  480  000 

0 

o 

0 

o 

"  21.. 

5,600 

800 

0 

3  200 

3  200 

12  010  400 

o 

o 

"  28.  ... 

14,400 

2,400 

100 

0 

0 

6  491  200 

o 

9  goo 

July   5  .  . 

8  800 

2  000 

160 

0 

0 

495  640 

o 

o 

"   12.. 

10,000 

2  400 

800 

400 

400 

3  900  920 

o 

400 

"   19 

12  800 

2  000 

0 

2  000 

2  000 

721  640 

o 

400 

"  26.  . 

2  400 

400 

0 

14  400 

14  400 

487  'OOO 

o 

o 

Aug.  2  .  . 

5,600 

800 

0 

17  600 

17  600 

4  474  400 

o 

o 

"   9  
"   16.  . 

2,800 
8  000 

400 
800 

0 
3  200 

78,400 
13  600 

78,400 
13  600 

69,000,200 
253  600 

0 

o 

0 

o 

"  23.  .  • 

12  800 

0 

2  400 

20  800 

20  800 

728  000 

o 

1  600 

"  30.  . 

6  400 

800 

800 

7  200 

7  200 

122  400 

o 

o 

Sept   6  .  . 

5  600 

o 

800 

4  800 

4  800 

120  001  640 

o 

800 

"   13.  . 

11  500 

0 

500 

500 

500 

1  451  120 

o 

9  000 

"  20.  .  . 

8  500 

500 

0 

18  000 

18  000 

1  923  'SOO 

o 

2  500 

"  27  

25,600 

1  600 

1  600 

65  000 

65  600 

851  400 

o 

o 

Oct.   4.  . 

8  000 

500 

500 

0 

0 

720  000 

o 

o 

"  11.  ... 

2  500 

1  000 

0 

0 

0 

843  540 

o 

3  500 

"   18  
"  25  

Nov.  1  .  . 
8  
"   15.. 

2,000 
15,000 

15,000 
5,000 
17  000 

1,000 
0 

1,000 
2,000 
2  000 

0 
60 

60 

2,000 
0 

500 
500 

0 
0 

o 

500 
500 

0 
0 

o 

1,744,000 
1,334,000 

985,560 
965,000 
488  100 

500 
500 

2,000 
0 
1  000 

5,000 
35,000 

31,000 

22,000 
28  000 

"  22... 

48  000 

8  000 

400 

0 

0 

750  640 

4  000 

108  000 

"  29  

200 

0 

200 

0 

0 

721  "00 

•  o 

47  500 

Dec.  6  .  . 

0 

0 

o 

o 

0 

720  580 

40 

7  000 

"  13.  ... 

0 

0 

0 

o 

0 

1  573  800 

100 

16  400 

"  20.  . 

0 

o 

o 

o 

o 

487  120 

o 

16  600 

"  27.  .  . 

200 

0 

o 

o 

o 

490  600 

200 

28  000 

Average  

7  194 

1  158 

368 

4  871 

4  760 

15  812  346 

630 

26  546 

327 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Codonella 
cratera 

Halteria 
grandinella* 

Stentor 
cceruleus 

Tintinnidium 
fluviatile 

Total 
Suctoria 

Metacineta 
mystacina 

2 

<i> 

11 
^ 

1 

£ 

Jan   11 

300 

80,000 

300 

0 

0 

0 

6,580 

0 

"  21  
"  25.  . 

300 
58,437 

40,000 
0 

28,800 
11,997 

0 
0 

100 
0 

0 
0 

49,240 
126^03 

0 
0 

Feb   3 

5,900 

0 

1,000 

0 

0 

0 

1  1  ,  496 

0 

"  is!  ! 

8,000 
5,200 

0 
0 

800 
800 

0 
0 

0 
0 

0 
0 

14,160 
31,040 

0 
0 

"  22  

15,795 

720,000 

0 

0 

0 

0 

48,649 

0 

Mar   1  .  . 

10,000 

0 

0 

0 

1,600 

1,600 

20,400 

400 

8,400 

0 

520 

0 

1,600 

1,600 

29,200 

800 

"   15 

33,200 

0 

1,000 

0 

400 

400 

103,940 

400 

"  22 

41  ,600 

60,000 

80 

0 

1,200 

1,200 

185,520 

400 

"  29  
Apr   5 

30,400 
20,500 

0 
0 

0 
-   20 

0 
300 

200 
100 

200 
0 

115,880 
84,820 

5,020 
1,800 

"  12  . 

20,100 

900,000 

0 

200 

100 

0 

54,540 

0 

"   19  
"  26  

453,600 
614,400 

0 
0 

0 
0 

400 
12,800 

0 
0 

0 
0 

749,000 
2,892,360 

0 
4,800 

May   3  .  . 
"   10.  . 

736,000 
78,400 

0 
0 

0 
0 

720,000 
24,000 

0 
0 

0 
0 

5,247,800 
2,663,400 

0 
200 

"   17 

72  000 

0 

0 

10,400 

800 

800 

1,465,500 

800 

"  24 

74,200 

0 

0 

400 

0 

0 

196,020 

3,200 

"  31  
June  7 

61,200 
1  499  200 

0 
60  000 

0 
0 

400 
14,400 

200 
0 

200 
0 

180,760 
903,000 

18,800 
392,000 

"   14  . 

532  ,800 

0 

0 

104,000 

0 

0 

639,600 

1,600 

"  21  .. 

195,  ?00 

0 

0 

74,400 

800 

0 

2,601,200 

3,200 

"  28  

45,600 

3,600,000 

0 

33,600 

0 

0 

1,118,400 

0 

July   5  
"   12.  . 

13,600 
35,600 

0 

2  700,000 

0 
0 

4,800 
5,600 

7,200 
400 

7,200 
400 

153,000 
184,500 

800 
0 

"   19  

24,000 

0 

0 

2,800 

400 

400 

946,080 

0 

"  26 

2,000 

120,000 

0 

3,600 

•0 

0 

370,200 

0 

Aug   2  .  . 

23,200 

1  800,000 

0 

95,200 

0 

0 

1,294,240 

0 

9  
"   16 

8,400 
20,000 

0 
0 

0 
0 

4,800 
8,800 

0 
0 

0 
0 

782,720 
935,380 

0 
0 

"  23.  . 

26,400 

0 

0 

5,600 

1,600 

1,600 

696,180 

1,600 

"  30  

51,200 

0 

0 

800 

0 

0 

435,080 

1,600 

Sept  6  .  . 

13,600 

0 

40 

0 

0 

0 

422,840 

0 

"  13.  . 

49,000 

0 

120 

2,000 

0 

0 

197,960 

0 

"  20  

34,500 

0 

0 

20,000 

500 

0 

475,860 

1,000 

"  27 

92  800 

0 

200 

22,400 

0 

0 

1,792,700 

14,400 

Oct.  "4.  . 

23,000 

0 

0 

1,500 

0 

0 

105,020 

2,580 

"   11 

23  000 

0 

40 

500 

0 

0 

122,000 

2,000 

"   18.  . 

47,000 

900,000 

0 

1,000 

40 

40 

159,200 

0 

"  25.  . 

23,000 

0 

0 

0 

0 

0 

1,048,620 

0 

Nov.  1  .  . 
8.  . 

12,500 
70  000 

0 
0 

60 
0 

0 
0 

0 

0 

0 
0 

156,300 
147,780 

0 
0 

"   15.  ... 

35,000 

0 

100 

0 

0 

0 

180,600 

0 

"  22  
"  29.  . 

2,000 
2  000 

0 
0 

0 
0 

0 
0 

0 
0 

0 
0 

128,400 
66,000 

0 
0 

Dec   6 

40 

0 

0 

0 

0 

0 

64,280 

0 

"  13.  . 

300 

1  080  000 

0 

0 

0 

0 

159,740 

0 

"  20  

200 

120,000 

0 

0 

0 

0 

191,320 

0 

"  27. 

200 

120  000 

o 

0 

0 

0 

50,540 

200 

Average  

101,024 

255,769 

882 

22,590 

332 

301 

592,416 

328 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Conochilus 
dossuarius 

Conochilus 
unicornis 

Total 
Bdelloida 

Philodina 
megalotrocha 

Rotifer 
neptunius 

11 

'S  2 

CS 

« 
.| 

"*.§ 

fr; 

Anuraa 
aculeata 

Jan.  11  .. 

0 

0 

400 

0 

0 

400 

6,180 

0 

"  21 

0 

0 

45  100 

0 

0 

44  500 

4  040 

0 

"  25.  . 

0 

0 

90,171 

0 

0 

89,379 

35  271 

0 

Feb   3 

0 

0 

3  800 

0 

0 

3  800 

7  696 

0 

8  
"  15.  . 

0 
0 

0 
0 

6,800 
18,000 

0 
0 

0 
0 

6,800 
27,000 

7,360 
12  240 

0 
0 

"  22 

0 

0 

25  272 

0 

0 

25  272 

23  377 

0 

Mar   1  .  . 

0 

0 

1,600 

0 

400 

800 

18  320 

0 

8  

400 

0 

4,040 

0 

40 

4,000 

23,960 

400 

"   15 

0 

400 

22  160 

80 

400 

19  200 

80  980 

40 

"  22  
"  29.  .. 

0 
0 

400 
20 

10,440 
1,620 

0 
0 

40 
20 

10,400 
1,600 

174,680 
109,240 

400 
200 

Apr.   5  .  . 
"   12.  .. 

0 
0 

0 
0 

1,100 
960 

0 
0 

0 
60 

1,100 
800 

81,920 

53,480 

600 
600 

"   19 

0 

400 

3  300 

400 

100 

16  000 

745  300 

2  000 

"  26.  . 

0 

3,200 

4,640 

0 

640 

3,200 

2  889  720 

3  200 

May   3 

0 

0 

16  000 

0 

0 

12  800 

5  231  800 

22  400 

"   10  
"   17  

"  24.  .  . 

0 
0 
0 

200 
800 
3,200 

14,400 
20,800 
1,040 

0 
0 

80 

1,600 
6,400 
520 

11,200 
10,400 
400 

2,647,200 
1,438,300 
191  780 

35,600 
22,400 
4  000 

"  31 

o 

18  600 

880 

80 

200 

600 

161  080 

1  400 

June  7  .... 
"   14  

0 
0 

392,000 
1,600 

800 
600 

0 
0 

800 
400 

0 
200 

507,000 
637,400 

1,600 
800 

"  21 

3  200 

0 

1  100 

0 

300 

800 

2  593  600 

0 

"  28.  . 

0 

0 

1  900 

0 

800 

300 

1  112  500 

0 

July   5 

0 

800 

2  480 

80 

1  600 

800 

146  920 

o 

"   12  
"   19.  . 

0 
0 

0 
0 

4,800 
2  760 

400 
1  600 

2,000 
360 

2,400 
800 

178,100 
933  320 

0 
0 

"  26.  .. 

0 

0 

120 

0 

0 

60 

268,480 

0 

Aug   2 

0 

0 

1  400 

0 

560 

40 

1  260  840 

o 

9.  . 

0 

0 

1  200 

0 

0 

12  000 

775  920 

o 

"   16.  .. 

0 

0 

4  120 

0 

120 

4  000 

907  260 

o 

"  23  

1,600 

0 

5,720 

0 

60 

5,600 

671  ,260 

0 

"  30 

1  600 

o 

4  080 

0 

80 

2  400 

415  000 

o 

Sept.  6  ... 

0 

0 

9  640 

2  400 

40 

4  800 

413  200 

o 

"  13  

0 

0 

21,000 

500 

1,500 

17,500 

171,960 

0 

"  20.  . 

1  000 

o 

6  000 

1  500 

500 

3  000 

460  360 

o 

"  27.  . 

14  400 

o 

13  300 

8  000 

300 

4  800 

1  744  200 

o 

Oct   4 

2  500 

o 

2  280 

2  000 

120 

160 

97  160 

o 

"  11  .. 

2  000 

o 

2  000 

1  000 

500 

500 

115  SOO 

o 

"   18.  . 

0 

o 

540 

0 

40 

0 

188  160 

o 

"  25  

0 

0 

3,500 

0 

1  000 

2,500 

1,045  120 

o 

Nov.  1  .  . 

0 

o 

3  060 

0 

60 

3  000 

152  680 

o 

8.  .  . 

0 

0 

1  180 

120 

60 

1  000 

146  600 

0 

"  15. 

0 

o 

100 

0 

100 

0 

180  500 

o 

"  22.  . 

0 

o 

400 

0 

400 

0 

126  000 

o 

"  29.. 

0 

o 

0 

0 

0 

0 

66  000 

o 

Dec.   6  .  . 

0 

o 

20 

0 

0 

20 

64  260 

o 

"  13.. 

0 

o 

600 

0 

0 

600 

159  140 

o 

"  20.  ... 

0 

0 

0 

0 

0 

0 

191  320 

0 

"  27.. 

200 

o 

20 

o 

20 

o 

50  120 

20 

Average  

517 

8  108 

405  983 

351 

425 

6  688 

571  611 

1  839 

329 


TAB-LE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Anurcea 
cochlearis 
type  and  var. 
macracantha 

Anurcea 
cochlearis 
var.  stipitata 

Anurcea 
cochlearis 
var.  tecta 

Total 
Anurcea 
cochlearis 

Total  eggs 
Anurcea 
cochlearis 

Atmrcea 
hypelasma 

Total  eggs 
Anurcea 
hypelasma 

Jan   11.. 

0 

0 

0 

0 

0 

100 

100 

"  21  

300 

0 

0 

300 

100 

0 

0 

"  25 

387 

0 

1,661 

2,048 

0 

0 

0 

Feb   3  .  . 

500 

0 

100 

600 

300 

0 

0 

8 

0 

0 

80 

80 

0 

0 

0 

"   IS.  . 

80 

0 

0 

80 

0 

0 

0 

"  22.  . 

0 

0 

0 

0 

0 

0 

0 

Mar   1  . 

400 

0 

80 

480 

0 

0 

0 

8.  . 

800 

0 

1,200 

2,000 

1,600 

0 

0 

"   IS.  .  . 

2,200 

0 

600 

2,800 

1,400 

0 

0 

"  22 

3  200 

0 

800 

4,000 

2,800 

0 

0 

"  29. 

2,600 

0 

600 

3,200 

200 

0 

0 

Apr.   5  .  . 

0 

1,700 

400 

2,100 

600 

0 

0 

"   12 

1  800 

0 

400 

2,200 

800 

0 

0 

"   19.  . 

12,400 

0 

2,800 

15,200 

8,800 

0 

400 

"  26.  . 

12,800 

121,000 

4,000 

137,800 

57,800 

0 

0 

222  400 

745,600 

54,400 

1,022,400 

552,200 

0 

0 

"   10.  . 

134,400 

790,400 

220,800 

1,145,600 

643,200 

0 

0 

"   17.  . 

91  ,200 

295,600 

48  ,  000 

434,800 

160,000 

0 

0 

"  24 

1  000 

18  400 

1,800 

21,200 

7,200 

0 

0 

"  31  . 

1  400 

9,200 

600 

11,200 

3,400 

0 

0 

June  7  .... 

0 

32,000 

0 

32,000 

3,200 

0 

0 

"  14  

0 

28,000 

1,600 

29,600 

7,800 

0 

2,400 

"  21  .. 

0 

150,400 

222,400 

372,800 

148,800 

9,600 

8,800 

"  28.  . 

0 

48,800 

117,600 

166,400 

20,800 

7,200 

4,000 

July   5.  . 

0 

2,800 

7,200 

10  000 

1,600 

800 

400 

"   12.  . 

0 

2,000 

8,000 

10,000 

4,000 

1,200 

0 

"   19.  . 

0 

2,000 

15,200 

17,200 

5,600 

4,000 

0 

"  26 

0 

0 

1  200 

1  200 

0 

0 

0 

Aug   2  .  . 

0 

0 

0 

0 

0 

4,800 

2,400 

9.  . 

0 

0 

0 

0 

0 

2,000 

4,000 

"   16 

0 

0 

0 

0 

0 

16  000 

8  800 

"  23.  . 

0 

0 

0 

0 

0 

9  600 

3,200 

"  30.  . 

0 

0 

0 

0 

0 

800 

800 

Sept   6 

0 

0 

0 

0 

0 

0 

0 

"   13.  . 

0 

500 

0 

500 

0 

1  ,000 

0 

"  20.  . 

0 

3,500 

8,500 

12,000 

6,000 

4,000 

1,000 

"  27  

0 

19,200 

35,200 

54,400 

16,000 

43,200 

54,400 

Oct   4.  . 

0 

4,000 

2,000 

4,000 

500 

2,000 

500 

"   11  ... 

0 

7,000 

2,000 

9,000 

4,500 

500 

0 

"   18 

o 

17  500 

7  000 

24  500 

10  500 

3  500 

2,500 

"  25.  . 

500 

9,000 

19,000 

28,500 

7,000 

13,500 

5,000 

Nov.  1  .  . 

500 

0 

1,000 

1,500 

,0 

500 

1  ,000 

8  
"  15.  . 

0 
0 

0 
0 

0 
0 

0 
0 

0 
0 

0 
0 

0 
0 

"  22.  ... 

0 

0 

0 

0 

0 

0 

0 

"  29  

500 

1,000 

8,500 

10,000 

500 

0 

0 

Dec.   6  .  . 

0 

1  ,  000 

1,020 

2,200 

20 

0 

0 

"   13.  . 

500 

1  700 

5  100 

7  300 

1  800 

0 

0 

"  20.  .. 

0 

3  600 

1  ,600 

5,200 

2,600 

0 

0 

"  '  27 

0 

200 

0 

200 

0 

0 

0 

Average  

9,421 

44  540 

15,432 

69,165 

32,358 

2,390 

1,917 

330 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Asplanchna 
brightwellii 

Asplanchna 
priodonta 

Brachionus 
angularis 

Brachionus 
angularis 
var.  bidens 

Total 
Brachionus 
angularis 

Total  eggs 
Brachionus 
angularis 

Brachionus 
bakeri 
var. 
brevispinus 

Jan  1  1  . 

0 

0 

0 

0 

0 

0 

0 

"  21  .. 

100 

0 

0 

0 

0 

0 

0 

"  25  

0 

0 

387 

0 

387 

0 

0 

Feb   3  .  . 

0 

0 

0 

0 

0 

0 

0 

8  

0 

0 

0 

0 

0 

0 

0 

"   IS 

o 

0 

0 

0 

0 

0 

0 

"  22.  . 

0 

0 

0 

0 

0 

0 

0 

Mar   1 

80 

0 

0 

0 

0 

0 

0 

8.  . 

0 

0 

0 

0 

0 

0 

0 

"   IS  

0 

0 

0 

0 

0 

0 

0 

"  22  

0 

0 

0 

0 

0 

0 

40 

"  29  

0 

0 

0 

0 

0 

0 

0 

Apr   5  

20 

0 

100 

0 

100 

0 

0 

"   12  

0 

0 

0 

0 

0 

0 

0 

"   19.  . 

0 

0 

0 

0 

0 

0 

0 

"  26.  .. 

0 

0 

0 

0 

0 

0 

0 

May   3  .  . 

16  000 

0 

0 

0 

0 

0 

0 

"   10.  . 

20,800 

3,200 

1,600 

0 

1,600 

0 

0 

"   17  

11  ,200 

14,400 

0 

800 

800 

800 

0 

"  24  
"  31  .. 

400 
200 

2,120 
2,000 

0 
1,400 

200 
0 

200 
1,400 

0 
0 

0 
0 

June  7  .  . 
"   14  
"  21  .  . 

200 
0 
1  ,100 

0 
0 
1,100 

4,800 
4,000 
70,400 

0 
0 
0 

4,800 
4,000 
70,400 

0 
1,600 

24,800 

0 
0 
0 

"  28  

100 

0 

544,000 

0 

544,000 

128,800 

0 

July   5  . 

160 

0 

29,200 

400 

29,600 

1,600 

0 

"  12.  . 

0 

0 

51,200 

0 

51,200 

13,200 

400 

"  19  

280 

0 

300,800 

34,800 

335,600 

72,800 

0 

"  26 

17  900 

0 

6  400 

10,400 

16,800 

1,200 

0 

Aug.  2  ... 

23,200 

0 

10,400 

93,600 

103,200 

12,000 

0 

9  

80 

0 

229,200 

64,800 

292,600 

105,600 

400 

"   16  

800 

0 

272,800 

80,800 

353,600 

116,000 

0 

"  23.  . 

4,000 

0 

77,600 

138,400 

216,000 

42,400 

0 

"  30  

2,400 

0 

28,800 

86,400 

115,200 

28,000 

2,400 

Sept   6.  . 

0 

0 

80,000 

83,200 

163,200 

35,200 

400 

"   13.  . 

0 

60 

27,000 

10,000 

36,500 

18,000 

2,000 

"  20  

1,140 

60 

87,500 

27,500 

115,000 

43,000 

0 

"  27  

6,400 

0 

409  ,  600 

84,800 

494,400 

41,600 

1,600 

Oct.   4.  . 

500 

0 

19,000 

9,000 

28,000 

2,000 

0 

"  11  

1,000 

0 

8,000 

1,000 

9,000 

2,000 

0 

"  18  

0 

0 

8,000 

500 

8,500 

2,500 

0 

"  25.. 

60 

0 

11,500 

0 

11,500 

5,500 

0 

Nov.  1  .  . 

0 

0 

1,000 

,  0 

1,000 

0 

0 

8.  ... 

0 

0 

0 

0 

0 

0 

0 

"  IS... 

0 

0 

100 

0 

100 

0 

0 

"  22  

0 

0 

0 

0 

0 

0 

0 

"  29  

0 

0 

0 

0 

0 

0 

0 

Dec.  6  .  . 

0 

0 

20 

0 

20 

0 

0 

"  13  

0 

0 

0 

0 

0 

0 

0 

"  20.  . 

0 

0 

400 

0 

400 

0 

0 

"  27  

0 

0 

0 

0 

0 

0 

0 

2  079 

441 

43  946 

13,973 

57,919 

13,242 

139 

331 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Brachionus 
bakeri 
var.  clunior- 
bicularis 

Brachionus 
bakeri 
var.  melhemi 

Brachionus 
bakeri 
var.  obesus 

Brachionus 
bakeri 
var.rhenanus 

Brachionus 
bakeri 
var. 
tuberculus 

Total 
Brachionus 
bakeri 

§ 
II 

*|'l 

d  g^ 

ocqj 
H 

Jan      11.. 

0 

0 

0 

0 

0 

0 

0 

"     21    . 

0 

0 

0 

0 

0 

0 

0 

"     25.  . 

0 

0 

0 

0 

0 

0 

0 

Feb       3 

0 

0 

0 

0 

0 

0 

0 

8 

0 

0 

0 

0 

0 

0 

0 

"      IS.  . 

0 

0 

0 

0 

0 

0 

0 

"     22  

0 

0 

0 

0 

0 

0 

0 

Mar       1    . 

0 

0 

0 

0 

0 

0 

0 

8.  . 

0 

0 

0 

0 

0 

0 

0 

"      IS  

0 

0 

0 

0 

0 

0 

0 

"     22 

0 

0 

0 

0 

0 

40 

0 

"     29 

0 

0 

0 

0 

0 

0 

0 

Apr       5  .  . 

0 

0 

0 

0 

0 

0 

0 

"      12 

0 

0 

0 

0 

0 

0 

0 

"      19    . 

0 

0 

0 

0 

0 

0 

0 

"     26.  . 

0 

0 

0 

0 

0 

0 

0 

May      3 

0 

0 

0 

0 

0 

0 

0 

"     10    . 

0 

0 

0 

0 

0 

0 

0 

"     17.  . 

0 

0 

0 

0 

0 

0 

0 

"     24  

0 

0 

0 

0 

0 

0 

0 

"     31 

40 

0 

0 

0 

0 

40 

0 

0 

0 

0 

0 

0 

0 

0 

"      14  

0 

0 

0 

0 

0 

0 

0 

"     21 

0 

0 

0 

0 

0 

0 

0 

"     28.  . 

800 

0 

0 

0 

100 

900 

0 

Tulv      5 

400 

0 

0 

0 

400 

800 

400 

•'      12! 

0 

60 

0 

0 

1,200 

1,660 

60 

"      19.  ... 

920 

1,200 

40 

0 

0 

2,160 

2,520 

"     26  

0 

0 

0 

0 

0 

0 

0 

Aug      2  .  . 

0 

0 

0 

0 

0 

0 

0 

9.  ... 

400 

0 

0 

40 

0 

840 

800 

"      16  

800 

0 

0 

1,600 

0 

2,400 

5,600 

"     23  

0 

0 

0 

0 

0 

0 

0 

"     30.  . 

0 

800 

800 

800 

800 

5,600 

2,400 

Sept.     6  

800 

0 

800 

1,600 

4,000 

7,600 

5,600 

"      13  

500 

0 

0 

2,000 

0 

4,500 

4,000 

"     20.  . 

0 

500 

0 

0 

0 

500 

500 

"     27.. 

0 

0 

0 

0 

1,600 

3,200 

0 

Oct       4    . 

40 

0 

0 

0 

0 

0 

0 

"      11  .. 

0 

0 

0 

0 

0 

0 

0 

"      18.  . 

0 

0 

0 

40 

0 

40 

0 

"     25  

0 

0 

500 

0 

0 

500 

0 

Nov      1  .  .    . 

0 

0 

0 

60 

0 

60 

0 

8  

0 

0 

0 

0 

0 

0 

0 

"     IS. 

0 

0 

0 

0 

0 

0 

0 

"     22.  . 

0 

0 

0 

0 

0 

0 

0 

"     29  

0 

0 

0 

0 

0 

0 

0 

Dec.      6.  . 

0 

0 

0 

0 

0 

0 

0 

"     13  

0 

0 

0 

0 

0 

0 

0 

"     20.  . 

0 

0 

0 

0 

0 

0 

0 

"     27.  .. 

0 

0 

0 

0 

0 

0 

0 

Average  

90 

49 

41 

118 

155 

592 

420 

332 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Brachionus 
budapesti- 
nensis 

Brachionus 
militaris 

Brachionus 
mollis 

Brachionus 
pala 

Brachionus 
pala 
var. 
amphiceros 

Brachionus 
pala 
var.  dorcas 

Brachionus 
pala  var. 
dorcas  forma 
spinosus 

Total 

Brachionus 
pala 

Jan.  1  1  ... 
"  21.  . 

0 

o 

0 

o 

0 

o 

20 
100 

20 

o 

20 

o 

0 

o 

60 
100 

"  25.  . 

0 

o 

o 

o 

o 

387 

0 

387 

Feb.  3  .  . 

0 

o 

o 

o 

o 

200 

o 

200 

8.  . 

o 

o 

o 

o 

o 

o 

o 

o 

"  IS.  .  . 

0 

o 

o 

o 

o 

o 

o 

o 

"  22  

0 

o 

o 

o 

o 

o 

o 

o 

Mar   1  .  . 

0 

o 

o 

80 

o 

o 

o 

80 

8  

0 

o 

o 

0 

o 

80 

o 

80 

"  15  

"  22.  . 

0 

o 

0 

o 

0 

o 

160 

o 

0 

o 

360 

1  720 

0 

o 

520 
1  720 

"  29  

o 

o 

o 

200 

o 

140 

o 

340 

Apr.   5  .  . 
"  12.  .. 

0 

o 

0 

o 

0 

o 

0 

200 

20 
120 

100 
160 

0 

o 

120 
480 

"   19  

•     0 

o 

o 

2  800 

1  200 

800 

o 

4  goo 

"  26.  . 

o 

o 

o 

57  920 

97  600 

4  000 

o 

159  520 

May  3  .  . 

o 

o 

32  000 

419  200 

o 

o 

451  200 

"  10  . 

o 

o 

o 

19  200 

57  600 

o 

o 

76  800 

"   17.  . 

o 

o 

o 

5  600 

69  600 

800 

1  700 

77  700 

"  24.  . 

o 

200 

o 

80 

200 

o 

o 

280 

"  31  
June  7  .  . 

0 

o 

0 

o 

0 

o 

0 
200 

0 

o 

0 

o 

0 

o 

0 
200 

"  14  

o 

o 

o 

o 

o 

o 

o 

1  000 

"  21  
"  28. 

0 

4  000 

0 

o 

0 

o 

800 

o 

200 

o 

0 

o 

0 

o 

0 

o 

July   5  .  . 

3  600 

40 

o 

40 

o 

o 

o 

40 

"  12  

10  000 

120 

o 

0 

o 

o 

o 

o 

"   19.  . 

85  600 

80 

o 

800 

5  600 

o 

o 

6  400 

"  26.  . 

3  200 

o 

o 

o 

120 

o 

o 

120 

Aug.  2  .  . 

9  600 

0 

40 

o 

6  400 

o 

o 

6  400 

9.. 

11  200 

o 

200 

1  200 

2  000 

o 

o 

3  200 

"   16.  . 

20  000 

o 

800 

800 

37  600 

o 

o 

38  400 

"  23  

8  000 

800 

o 

o 

35  200 

o 

o 

35  200 

"  30  

7  200 

3  200 

800 

800 

32  000 

o 

o 

32  800 

Sept.  6.  . 

7  200 

1  600 

o 

o 

19  200 

o 

o 

19  200 

"  13  

1  500 

o 

o 

500 

4  000 

o 

o 

4  500 

"  20  

2  000 

0 

500 

500 

9  000 

o 

o 

9  500 

"  27.  . 

44  800 

1  600 

4  800 

4  800 

78  400 

o 

o 

83  200 

Oct.   4  .  . 

80 

0 

o 

40 

1  000 

o 

o 

1  040 

"  11  

0 

0 

0 

500 

o 

o 

o 

500 

"  18  
"  25.. 

1,000 
0 

0 

o 

0 

o 

500 
3  500 

80 
5  000 

0 

o 

0 

o 

580 
8  500 

Nov.  1  .  . 
8  
"  15.. 

0 
0 

o 

0 
0 

o 

0 
0 

o 

0 
0 
1  000 

0 
180 
100 

0 
0 

o 

0 
0 

o 

0 
180 
1  100 

"  22... 

o 

o 

o 

400 

400 

o 

o 

800 

'  29  
Dec.   6.  . 

0 

o 

0 

o 

0 

o 

1,000 
320 

500 
1  160 

0 
20 

0 

o 

1,500 
1  500 

"   13  
"  20.  ... 
"  27.. 

0 
0 

o 

0 
0 

o 

0 
0 

o 

2,400 
1,200 
400 

3,200 
606 
200 

0 
0 
40 

0 
0 

o 

5,600 
1,800 
640 

Average  

4  211 

147 

137 

2  693 

17  071 

170 

33 

19  969 

333 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Total  eggs 
Brachionus 
pala 

Brachionus 
urceolaris 

Brachionus 
urceolaris 
var. 
bursarius 

Brachionus 
urceolaris 
var.  rubens 

Total 
Brachionus 
urceolaris 

Total  eggs 
Brachionus 
urceolaris 

Brachionus 
variabilis 

Brachionus 
free  winter 
eggs 

Jan.  11.. 
"  21.  . 

100 
100 

0 
0 

0 
0 

0 

40 

0 

40 

0 
0 

0 
0 

100 
0 

"  25.. 

1,161 

0 

0 

0 

0 

0 

0 

1,548 

Feb   3 

0 

0 

0 

0 

0 

0 

0 

100 

8.  . 

0 

0 

0 

0 

0 

0 

0 

0 

"   15.  .. 

800 

0 

0 

0 

0 

0 

0 

2,400 

"  22  

632 

0 

0 

0 

0 

0 

0 

3,791 

Mar   1  .  . 

480 

0 

0 

80 

80 

0 

0 

480 

8.  ... 

160 

0 

0 

0 

0 

0 

0 

840 

"   15  

1,000 

0 

0 

160 

160 

0 

0 

400 

"  22  

2,920 

0 

0 

2,000 

2,000 

400 

0 

440 

"  29. 

420 

0 

0 

1,800 

1,800 

1,200 

0 

20 

Apr.   5  .  . 

20 

0 

0 

700 

700 

400 

0 

120 

"   12 

240 

0 

0 

140 

140 

60 

0 

200 

"   19 

5,200 

0 

0 

400 

400 

0 

0 

0 

"  26.  . 

324,280 

0 

0 

6,400 

6,400 

0 

0 

0 

May  3 

661,200 

0 

0 

0 

0 

0 

0 

41,600 

"  10. 

118,400 

0 

0 

0 

0 

0 

0 

9,600 

"   17.  . 

101,700 

0 

0 

0 

0 

0 

0 

800 

"  24..  . 

1,040 

0 

0 

0 

0 

0 

200 

80 

"  31 

0 

0 

0 

0 

0 

0 

0 

400 

June  7  .  . 

0 

0 

0 

0 

0 

0 

0 

0 

"   14.  ... 

400 

0 

0 

0 

0 

0 

0 

400 

"  21 

100 

800 

0 

0 

800 

0 

0 

0 

"  28.  . 

100 

100 

0 

0 

100 

0 

0 

100 

July   5  .  . 

40 

0 

0 

0 

0 

0 

0 

40 

"   12  . 

400 

0 

0 

0 

0 

0 

0 

1,200 

"   19.  . 

400 

40 

200 

0 

240 

0 

0 

400 

"  26.  .. 

800 

0 

400 

0 

400 

0 

0 

1,200 

Aug   2 

1,200 

0 

0 

0 

0 

o" 

0 

0 

9.  . 

8,400 

0 

800 

0 

800 

0 

0 

0 

"   16.  .. 

5,600 

0 

800 

0 

800 

0 

0 

800 

"  23  
"  30  

14,400 
12,000 

0 
0 

2,400 
0 

0 
0 

2,400 
0 

800 
0 

0 
0 

4,800 
0 

Sept.  6.  .  . 

22,400 

0 

5,600 

0 

5,600 

0 

0 

800 

"   13  
"  20  
"  27.  . 

3,000 
5,500 
32,000 

0 
0 
0 

500 
0 
0 

0 
0 
0 

500 
0 
0 

0 
0 

0 

0 
0 
0 

0 
500 
.  1,600 

Oct   4 

500 

0 

0 

0 

0 

0 

0 

500 

"   11 

0 

0 

0 

0 

0 

0 

0 

500 

"   18.  . 

500 

0 

0 

0 

.  0 

0 

0 

540 

"  25  

4,500 

0 

0 

0 

0 

0 

0 

500 

Nov   1  .  . 

0 

0 

0 

0 

0 

0 

0 

2,000 

8.  . 

120 

0 

0 

0 

0 

0 

0 

2,000 

"  15  
"  22.  . 

1,200 
2,400 

0 
0 

0 
0 

0 
0 

0 
0 

0 
0 

0 
0 

1,000 
2,000 

"  29.  . 

1,500 

0 

0 

0 

0 

0 

0 

500 

Dec   6  .  . 

860 

0 

0 

100 

100 

0 

0 

0 

"  13.  .. 

10,600 

0 

0 

600 

600 

0 

0 

0 

"  20  

1,800 

0 

0 

40 

40 

0 

0 

0 

"  27.  . 

100 

0 

0 

240 

240 

80 

0 

1,621 

25  974 

18 

206 

244 

468 

56 

4 

1,685 

334 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Mastigocerca 
carinata 

Metopidia 
solidus 

Monostyla 
bulla 

Monostyla 
lunaris 

Notholca 
striata  var. 
acuminata 

Polyarthra 
platyptera 

Polyartha  platyptera 
Male  eggs 

Free 

Carried 

Jan     11 

0 
0 
0 

0 
0 
0 
0 

400 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
19,200 
11,200 

2,000 
1,600 
7,200 
4,000 

15,200 
4,000 
5,600 
5,600 
800 

0 
2,500 
1,500 
4,800 

1,000 
0 
0 
0 

60 
0 
0 
400 
0 

0 
0 
0 
0 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
400 
0 

0 
100 
400 
0 

0 
0 
800 
0 
0 

0 
0 
800 
800 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
200 
0 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

100 
100 
400 
0 

0 
0 
0 
200 
200 

0 

•    o 

0 
0 

0 
0 
800 
800 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
100 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
800 
400 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
500 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 

24 
240 
80 
0 

800 
1,200 
6,400 
10,800 
200 

300 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
40 
0 

1,000 
1,200 
11,997 

3,200 
2,000 
1,600 
6,318 

3,200 
5,200 
22,200 
37,600 
40,400 

42,800 
26,700 
148,200 
696,000 

582,400 
137,600 
195,200 
52,200 
52,400 

304,000 
432,800 
241,600 
56,800 

6,400 
21,600 
89,200 
86,400 

288,000 
55,200 
84,800 
96,000 
51,200 

4,000 
31,000 
72,500 
238,400 

24,500 
47,500 
27,000 
37,500 

500 
1,000 
2,000 
6,000 
1,000 

6,020 

42,100 
63,400 
19,200 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

1,300 
900 
8,800 
150,400 

0 
4,800 
12,000 
0 
200 

1,600 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 

5,000 
2,000 
0 

0 
0 
0 
0 
0 

0 
100 
0 
0 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
1,600 
2,600 

2,800 
1,900 
1,600 
53,800 

19,200 

0 
2,400 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
1,600 

0 
500 
1,500 
2,000 

0 
0 
0 
0 
0 

160 
100 
200 
0 

"     21 

"     25.  . 

Feb       3 

8. 

"     15.. 

"     22 

Mar       1  .  . 

8  

"     15 

"     22.  . 

"     29.  . 

Apr       5    . 

"      12.  . 

19 

"     26 

May      3  .  . 

"      10 

"      17  
"     24.  . 

"     31 

June     7  .  . 

"     14.  . 

"     21    . 

"     28.  . 

July      5  .  . 

"      12 

"      19.  . 

"     26.  . 

Auf?      2 

"       9.  . 

"      16.  . 

"     23  

"     30.  . 

Sept.     6  .  . 

"      13  

"     20.  . 

"     27.  . 

Oct.      4.  ... 

"     11.  . 

"      18.  . 

"     25  

Nov.     1  .  . 

8  

"     15  

"     22.  . 

"     29  

Dec       6.  . 

"     13.  . 

"     20  

"     27.  . 

Average . 


1,674 


67 


50 


.57 


388 


86,674 


3,598  ; 


1,768 


335 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter^paper  collections.) 


1898 

Polyarthra 
platyptera 
Winter  eggs 

22 
It 

«£•" 

If  §8 

£*•" 

Pterodina 
patina 

Rattulus 
tigris 

Schizocerca 
diversicornis 

Synchceta 
pectinata 

Synchceta 
stylata 

Total  free 
winter  eggs 
Synchceta 

Free 

Carried 

Jan     11.. 

0 
100 
0 

0 
0 
800 
0 

0 
0 
200 
0 
200 

100 
0 
1,600 
22,400 

51,200 
11,200 
2,400 
400 
400 

0 
800 
800 
800 

400 
400 
800 
0 

0 
800 
0 
0 
800 

0 
1,000 
1,000 
1,600 

1,000 
0 
0 
0 

500 
1,000 
1,000 
0 
0 

0 
0 
0 
0 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

100 
100 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
1,600 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

2,200 
1  ,000 
8,127 

1,700 
2,800 
3,200 
6,318 

3,200 
4,000 
17,800 
26,400 
11,400 

10,400 
8,700 
104,200 
502,400 

316,800 
72,000 
120,000 
28,800 
10,600 

96,000 
119,200 
154,400 
16,000 

7,200 
13,600 
24,000 
53,600 

295,200 
84,800 
108,000 
63,200 
47,200 

8,800 
20,000 
103,000 
86,400 

15,000 
5,500 
14,000 
17,000 

4,500 
2,000 
2,000 
6,000 
3,000 

9,160 
29,300 
52,000 
1  1  ,  000 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
200 
40 

1,600 
0 
0 
100 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
100 
0 

0 
0 
400 
0 

0 
0 
40 
400 
100 

300 
200 
0 
0 

0 
3,200 
0 
0 
200 

0 
0 
0 
0 

40 
400 
0 
0 

800 
400 
800 
800 
1,600 

0 
0 
0 
0 

1,000 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 

800 
0 
0 
0 
800 

800 
0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
0 

.    0 
0 
0 
0 

0 
0 
0 

72 
0 
0 
632 

0 
0 
0 
0 
0 

0 
0 
400 
1,600 

0 
0 
800 
200 
600 

3,200 
800 
112,000 
0 

800 
400 
20,800 
400 

12,000 
4,800 
1,600 
3,200 
800 

0 
1,000 
4,500 
30,400 

500 
0 
500 
0 

0 
0 
0 
0 
500 

20 

2,500 
0 
400 

2,  \20 
300 
4,257 

1,000 
1,200 
800 
0 

6,400 
4,800 
15,200 
58,000 
47,000 

2  1  ,  000 
11,500 
368,000 
954,400 

1,139,000 
233,600 
206,400 
60,480 
61,600 

48,000 
19,200 
795,200 
22,400 

22,800 
9,600 
64,800 
8,000 

170,400 
52,000 
18,400 
24,800 
1,600 

0 
14,000 
27,000 
265,600 

5,000 
27,000 
77,000 
824,500 

110,500 
97,000 
110,000 
38,000 
39,000 

42  ,  500 
55,720 
59,200 
17,640 

100 
0 
0 

0 
0 
1,200 
0 

0 
0 
160 
0 
0 

0 
0 
0 
6,400 

9,600 
6,400 
800 
0 
0 

0 
800 
3,200 
0 

400 
800 
0 
0 

0 
800 
60 
0 
.  0 

0 
500 
500 
100 

0 
0 
0 
0 

0 
60 
0 
0 
0 

0 
0 
0 
0 

"     21 

"     25    . 

Feb       3  .  . 

8 

"     15.  . 

"     22.  . 

Mar       1 

8.  . 

"     15.  . 

"     22.  . 

"     29 

Apr       5  .  . 

"      12.  .. 

"     19 

••  26""" 

May      3  .  .     . 

"      10  

"      17.  . 

"     24.  . 

"     31  .. 

June     7  .  . 

"      14.  . 

"     21  ... 

"     28.  . 

July      5  .  . 

"      12.  ... 

"      19  

"     26.  . 

Aug.     2  .... 

9  

"      16.  . 

"     23.  . 

"     30.  . 

Sept      6  .  . 

"      13  
"     20... 

"     27  
Oct.      4.  . 

"      11  ... 

"      18.  ... 

"     25.. 

Nov.     1  .  . 

8.  . 

"     15.  . 

"     22.  .. 

"     29    . 

Dec.      6.  . 

"      13  

"     20.  . 

"     27.  .. 

Average  .  . 

1,994 

34 

52,560 

37 

207 

46 

3,950 

120,391 

611- 

336 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Total  eggs 
Synchceta 

Triarthra 
longiseta 

Triarthra 
longiseta 
eggs 

Pedalion 
mirum 

Total 
Entomostraca 

Total 
Ostracoda 

Total 
Cladocera 

Jan  11 

500 

0 

0 

0 

700 

0 

100 

"  21.. 

200 

0 

0 

0 

1  380 

0 

440 

"  25.  .. 

6,579 

0 

0 

0 

4  788 

0 

462 

Feb   3  .  . 

300 

0 

0 

0 

216 

0 

24 

8.  . 

400 

0 

0 

0 

320 

0 

0 

"   IS  

5,600 

0 

0 

0 

1  200 

80 

0 

"  22  
Mar   1  .  . 

0 
1,200 

0 

80 

0 
0 

0 
0 

3,285 
804 

0 
0 

0 
160 

8  

800 

0 

0 

0 

3  080 

40 

80 

"  IS.  . 

3  960 

0 

0 

0 

12  880 

40 

560 

"  22.  .. 

4  000 

40 

40 

0 

19  440 

320 

800 

"  29  

3,200 

100 

0 

0 

22  180 

160 

360 

Apr   5  .  . 

1,100 

300 

200 

o 

34  560 

200 

360 

"   12  

1,000 

200 

100 

o 

28  060 

320 

320 

"   19  
"  26.  . 

84,000 
38  400 

400 
3  200 

0 
0 

0 

o 

34,200 
56  800 

200 
800 

400 
1  920 

May  3  .  . 

278,400 

9,600 

0 

o 

204  800 

0 

2  800 

"  10 

91  600 

38  400 

o 

o 

235  400 

400 

5  600 

"  17.  . 

56  800 

17  600 

3  200 

o 

182  300 

1  600 

8  500 

"  24  

9,400 

600 

200 

o 

167  080 

400 

24  080 

"  31 

12  000 

1  000 

0 

o 

162  800 

440 

51  480 

June  7  .  . 

11  200 

200 

0 

o 

438  800 

1  600 

136  000 

"  14  

13,600 

0 

0 

o 

211  400 

400 

29  200 

"  21 

257  600 

800 

0 

100 

83  100 

200 

2  300 

"  28.  . 

51  200 

800 

0 

500 

45  600 

400 

10  100 

July  S  
"  12  
"   19.  . 

47,200 
16,800 
34  800 

400 
1,600 
4  000 

0 
400 
0 

1  ,  600 
1,200 
9  200 

4,920 
1,620 
14  040 

440 
60 

o 

640 
360 
1  240 

"  26  

4  800 

28  000 

1  600 

99  600 

23  000 

o 

9  960 

Aug.  2  .  . 

178,400 

18  400 

1  ,600 

30  400 

22  160 

40 

10  520 

"   9 

20  000 

4  400 

1  200 

4  000 

4  120 

o 

1  080 

"  16.. 

10  460 

3  200 

0 

22  400 

4*500 

800 

1  320 

"  23  

35,200 

4  000 

0 

17  600 

17  340 

180 

1  820 

."  30. 

7  200 

6  400 

o 

14*400 

27  080 

o 

4  040 

Sept   6  .... 

0 

0 

0 

0 

9  080 

0 

720 

"   13....'.. 

9,500 

1  000 

0 

5  000 

24  720 

SOO 

3  420 

"  20.  . 

17  500 

500 

o 

5  SOO 

21  880 

o 

1  560 

"  27.. 

38  500 

14  400 

3  200 

19  200 

99*300 

o 

1  100 

Oct.   4  .  . 

0 

1  500 

500 

2  000 

33  880 

0 

1  320 

"  11  .. 

2  000 

0 

o 

2  000 

34  060 

o 

2  000 

"   18.  . 

10  500 

1  500 

500 

500 

25  640 

0 

2  120 

"  25  

78  000 

500 

o 

o 

26*020 

0 

1  020 

Nov.  1  .  . 

29  000 

o 

o 

60 

8  600 

o 

120 

8  

41  060 

0 

0 

0 

IS  080 

0 

0 

"  IS  

60,000 

0 

0 

0 

13  100 

0 

100 

"  22  

66,000 

0 

0 

0 

13  920 

320 

2  800 

"  29.. 

500 

500 

500 

o 

6  180 

o 

80 

Dec.   6.  ... 

0 

0 

o 

0 

9  740 

0 

260 

"  13.  . 

800 

o 

o 

o 

21  740 

o 

240 

"  20  

2  600 

0 

o 

o 

2  440 

0 

400 

"  27  

400 

.  40 

0 

0 

6  800 

0 

280 

•Average  

31  620 

3  147 

255 

4  524 

47  042 

191 

6  241 

337 


TABLE     I  — ^continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

!« 

Bosmina 
longirostris 

Ceriodaphnia 
scitula 

Chydorus 
spharicus 

Daphnia 
cucullata 

S  2 

II 
tl 

a- 

Diaphanosoma 
brachyurum 

Moina 
micrura 

Jan     11 

0 

0 

0 

100 

0 

0 

0 

0 

"     21    . 

240 

0 

0 

200 

0 

0 

0 

0 

"     25  

154 

308 

0 

0 

0 

—  O, 

0 

0 

Feb       3 

0 

24 

0 

0 

0 

0 

0 

0 

8.  . 

0 

0 

0 

0 

0 

0 

0 

0 

"     15.  . 

0 

0 

0 

0 

0 

0 

0 

0 

"     22  

0 

0 

0 

0 

0 

0 

0 

0 

Mar       1  . 

0 

80 

0 

80 

0 

0 

0 

0 

8.  . 

0 

40 

0 

40 

0 

0 

0 

0 

"      15  

0 

120 

0 

440 

0 

0 

0 

0 

"     22  

0 

280 

0 

480 

0 

0 

0 

0 

"     29    . 

0 

20 

20 

240 

20 

0 

0 

0 

Apr       5  .  . 

20 

100 

40 

200 

0 

0 

0 

0 

"      12  

20 

60 

20 

200 

20 

0 

0 

0 

"      19 

200 

100 

0 

0 

0 

o 

0 

0 

"     26 

0 

800 

320 

800 

0 

0 

0 

0 

May      3  .  . 

0 

2,800 

0 

0 

0 

0 

0 

0 

"      10  

0 

3,600 

400 

600 

600 

0 

0 

0 

"     17  

200 

3,500 

400 

3,300 

300 

0 

0 

0 

"     24  
"     31.  . 

480 
40 

5,920 
33,920 

2,960 
8,720 

7,880 
5,040 

440 
1,000 

160 
720 

0 

40 

0 
0 

200 

62,800 

55,800 

600 

3,400 

11,600 

0 

0 

"      14 

0 

6,000 

10,600 

200 

2,400 

9   200 

0 

0 

"     21    . 

100 

1,500 

400 

0 

100 

0 

0 

200 

"     28.  . 

200 

700 

0 

0 

0 

0 

0 

100 

Tulv      5 

0 

200 

0 

40 

0 

0 

0 

400 

•'     12:: 

0 

180 

0 

0 

0 

0 

60 

120 

"      19.  . 

0 

0 

0 

160 

0 

0 

40 

1   040 

"     26 

0 

180 

120 

o 

o 

o 

8   580 

1   080 

Aug      2  .  . 

0 

40 

0 

0 

0 

0 

6  960 

3    520 

9.  . 

0 

0 

0 

0 

0 

0 

360 

360 

"      16  

0 

0 

0 

0 

0 

0 

60 

1   260 

"     23 

o 

0 

0 

0 

o 

0 

1   020 

900 

"     30 

0 

0 

40 

0 

0 

0 

2   520 

1   440 

Sept     6 

0 

0 

40 

o 

o 

o 

240 

440 

"      13.  ... 

0 

0 

0 

60 

0 

0 

1,800 

1    560 

"     20 

0 

60 

0 

60 

0 

o 

960 

480 

"     27.  . 

0 

0 

100 

0 

0 

o 

400 

500 

Oct       4 

0 

0 

0 

40 

400 

o 

880 

120 

"      11  . 

0 

920 

0 

0 

600 

o 

400 

40 

"      18.  . 

0 

1   360 

0 

80 

80 

o 

560 

0 

"     25.. 

0 

840 

0 

120 

60 

o 

0 

0 

Nov      1 

0 

60 

o 

60 

0 

o 

0 

0 

8.  . 

0 

0 

o 

0 

0 

o 

0 

0 

"     15.. 

0 

100 

o 

0 

0 

o 

0 

0 

"     22 

0 

o 

o 

o 

o 

o 

0 

0 

"     29. 

0 

0 

o 

80 

o 

o 

0 

0 

Dec.      6.  . 

0 

40 

20 

200 

0 

o 

0 

0 

"      13  
'     20.  . 

0 
0 

140 
120 

40 

o 

220 
160 

0 

o 

0 

o 

0 
0 

0 
0 

"     27.  . 

0 

0 

o 

280 

o 

o 

0 

o 

Average  

36 

2,441 

1,539 

422 

181 

417 

479 

261 

338 


TABLE      I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

Total 
Copepoda 

Canthocamptus 
spp. 

Cyclops 
albidus 

B 

ft.  =0 

O  S 

ft 

•^» 

-2§ 

^t3 
£* 

Cyclops 
prasinus 

Cyclops 
virdis  var. 
brevispinosus 

$ 

>2 

£  "  * 

IH§ 

"G-fe  s 
$** 

Jan  11.. 

600 

0 

0 

160 

o 

0 

0 

o 

"  21  

940 

40 

0 

160 

o 

0 

0 

40 

"  25 

4  326 

77 

0 

308 

o 

0 

0 

308 

Feb   3  ... 

192 

0 

0 

48 

o 

0 

0 

o 

8  

320 

0 

0 

160 

o 

0 

0 

o 

"  15.  . 

1  120 

0 

0 

80 

o 

0 

80 

o 

"  22.  . 

3  285 

0 

0 

0 

o 

0 

o 

o 

Mar   1 

644 

0 

0 

0 

o 

0 

o 

o 

8.  . 

2  960 

40 

0 

120 

o 

0 

0 

o 

"  15  

12  280 

40 

0 

400 

o 

0 

80 

120 

"  22  
"  29.  . 

18,320 
21  660 

200 
100 

0 
20 

80 
60 

0 

o 

0 
0 

0 
20 

80 

o 

Apr.   5  .  . 

34  000 

200 

20 

40 

0 

0 

0 

o 

"   12 

27  420 

1  120 

0 

0 

o 

0 

o 

40 

"   19.  . 

33  600 

0 

200 

200 

o 

0 

0 

500 

"  26  

54  080 

0 

1  600 

2  880 

o 

o 

0 

4  160 

May  3. 

202  000 

400 

0 

8  000 

400 

o 

0 

1  200 

"  10.  . 

229  400 

0 

600 

5  200 

o 

o 

600 

2  200 

"  17  

172  200 

800 

200 

600 

o 

o 

0 

3  300 

"  24  
"  31.  . 

142,600 
110  880 

80 
0 

920 
200 

320 
0 

0 
80 

0 

o 

1,080 
400 

1,640 
640 

301  200 

0 

400 

0 

0 

0 

2  600 

4  000 

"   14  

181,800 

0 

200 

0 

200 

0 

800 

4  400 

"  21  
"  28.  . 

80,600 
35  100 

0 

o 

0 
0 

0 

o 

0 

o 

0 

o 

0 

o 

400 
200 

July   S 

3  840 

o 

0 

o 

o 

o 

o 

120 

"   12.. 

1  200 

0 

0 

o 

o 

o 

o 

180 

"   19.  . 

12  800 

o 

0 

o 

40 

o 

40 

240 

"  26  

13  040 

o 

0 

o 

120 

o 

120 

300 

AUK   2  .  . 

11  600 

o 

0 

o 

0 

o 

0 

40 

AUK.  *.- 

3  040 

o 

0 

o 

80 

o 

0 

160 

"  16.  . 

2  380 

o 

o 

o 

0 

0 

0 

180 

"  23  

15,340 

o 

0 

0 

120 

0 

0 

660 

"  30.  . 

23  040 

o 

o 

o 

440 

o 

0 

480 

Sept   6  ... 

8  360 

o 

40 

0 

80 

0 

0 

0 

"  13  

20,800 

0 

0 

0 

120 

0 

0 

240 

"  20  

20,320 

0 

0 

0 

120 

0 

60 

360 

"  27.  . 

98  200 

100 

700 

o 

300 

o 

200 

700 

Oct.   4.  . 

32,560 

0 

120 

0 

320 

0 

200 

400 

"  11  

32,060 

0 

200 

0 

80 

0 

0 

120 

"   18.  . 

23  520 

o 

280 

o 

0 

o 

40 

40 

"  25.. 

25  000 

o 

60 

0 

60 

0 

120 

240 

Nov   1  .  . 

8  480 

o 

120 

o 

0 

o 

0 

60 

8.  . 

15  080 

o 

0 

o 

0 

0 

0 

120 

"  15  

13  000 

200 

0 

200 

0 

0 

0 

0 

"  22  

10  880 

480 

0 

80 

0 

0 

0 

160 

"  29.  . 

6  100 

80 

o 

4 

0 

o 

o 

0 

Dec.  6  ... 

9  480 

20 

0 

0 

0 

0 

20 

0 

"  13.  . 

21  500 

0 

o 

40 

0 

40 

0 

0 

"  20.  .  . 

2  040 

40 

o 

160 

0 

40 

0 

0 

"  27  

6  520 

0 

0 

120 

0 

40 

0 

0 

Average  

40,609 

78 

113 

373 

49 

2 

124 

539 

339 


TABLE     I  —  continued. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1908. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


1898 

&| 

II 

S* 

Copepodan 
nauplii 

ll 

«! 
?* 

Diaptomus 
siciloides 

a  a 

a* 

Total 
Nematodes 

Total 
Oligochsetes 

Chironomus 
larva 

Jan   11.. 

120 

240 

40 

0 

0 

0 

0 

0 

"  21  
"  25  . 

200 
770 

500 
2,709 

0 
0 

0 
0 

0 

o 

4,700 

774 

0 
77 

0 

o 

Feb   3  

72 

72 

0 

0 

0 

24 

100 

0 

"   8 

80 

80 

0 

0 

o 

80 

o 

o 

"   15  . 

160 

800 

0 

0 

0 

400 

o 

0 

"  22.  . 

126 

3,159 

0 

0 

0 

0 

126 

o 

Mar   1 

4 

640 

0 

0 

o 

0 

4 

o 

8.  . 

800 

2  000 

0 

0 

o 

400 

0 

40 

"   IS.  . 

220 

11  ,200 

40 

40 

o 

600 

o 

120 

"  22  

1  ,120 

16,800 

40 

0 

0 

40 

40 

40 

"  29 

760 

20  700 

0 

0 

o 

40 

20 

80 

Apr.   5  
"   12 

1,420 
1  100 

32,300 
25  100 

20 
60 

0 
0 

0 

o 

0 
40 

40 
40 

80 
160 

"   19  
"  26.  . 

5,500 
24,320 

27,200 
20  800 

0 
0 

0 

o 

0 

o 

100 
320 

100 
0 

300 
300 

May   3 

19  600 

182  400 

0 

o 

o 

o 

o 

400 

"  10 

53  200 

169  600 

0 

o 

o 

0 

200 

400 

"  17.  . 

27,900 

166  400 

0 

o 

900 

4  800 

200 

700 

"  24.  . 

12,160 

126  400 

0 

o 

840 

40 

80 

280 

"  31 

5  680 

103  800 

0 

o 

160 

40 

80 

440 

June  7  .  . 

23,800 

270  400 

0 

o 

0 

0 

200 

200 

"   14  
"  21 

1  1  ,  400 
14  600 

164,800 
65  600 

0 

o 

0 

o 

0 

o 

200 

o 

0 

o 

0 
400 

"  28.  . 

4  500 

30  400 

o 

o 

o 

300 

100 

400 

July   S  
"   12 

1,320 
780 

2,400 
180 

0 

o 

0 
60 

0 

o 

0 
0 

40 
180 

480 
300 

"   19.  . 

1  ,680 

10  800 

o 

60 

o 

0 

80 

480 

"  26.  . 

840 

11  600 

o 

0 

o 

o 

0 

120 

Aug   2  . 

360 

11  200 

o 

0 

80 

o 

40 

80 

9.  . 

400 

2  400 

o 

0 

0 

o 

0 

0 

"   16.  . 

600 

1  600 

o 

0 

0 

60 

60 

60 

"  23 

3  360 

11  200 

o 

o 

60 

60 

180 

o 

"  30  . 

5  520 

13  600 

o 

o 

0 

0 

120 

0 

Sept  6  .  .  . 

240 

8,000 

o 

o 

0 

40 

160 

80 

"   13 

4  320 

16  000 

60 

o 

o 

120 

180 

o 

"  20.  . 

0 

19  000 

o 

60 

0 

120 

60 

60 

"  27  
Oct   4 

0 
2  600 

96,000 
29  000 

0 

o 

200 

o 

0 

o 

300 
0 

400 
80 

200 

40 

"  11.  . 

3  920 

27  500 

o 

80 

o 

40 

40 

80 

"   18.  . 

5  580 

17  500 

40 

40 

o 

40 

40 

120 

"  25 

2  400 

22  000 

120 

o 

o 

120 

60 

o 

Nov.  1  ... 
8.  . 

300 
960 

8,000 
14,000 

0 
0 

0 
0 

0 

o 

0 
120 

180 
120 

0 
0 

"  15 

400 

12  000 

100 

0 

o 

100 

100 

0 

"  22.  . 

160 

10  000 

0 

0 

o 

2  000 

320 

0 

"  29.  . 

4 

6  000 

12 

0 

o 

0 

120 

0 

Dec   6.  . 

440 

9  000 

0 

o 

o 

500 

0 

0 

"   13.  ... 

1  ,060 

30  300 

60 

o 

0 

0 

0 

0 

"  20 

1  800 

0 

0 

20 

o 

o 

o 

•  o 

"  27.  . 

920 

5  400 

0 

0 

o 

20 

0 

40 

4  780 

36  707 

11 

10 

39 

318 

76 

124 

(23) 


340 


TABLE     I  —  concluded. 
ORGANISMS  PER  CUBIC  METER  IN  PLANKTON  OF  ILLINOIS  RIVER  IN  1898. 

(An  asterisk  at  head  of  column  indicates  that  all  entries  in  it  are  based  on 
filter-paper  collections.) 


J898 

li 

o^J 
H 

Plumatella 
statoblasts 

Total 
Glochidia 

Total 
Miscellaneous 

Total 
Phytoplank- 
ton 

Total 
Zooplankton 

! 
^ 

„  c 

cS  rt 

oE 
h 

Jan  11 

0 
0 
0 

0 
0 
0 
0 

0 
0 
0 
0 
6 

0 
0 
0 
0 

0 
0 
0 
0 
120 

200 
0 
300 
200 

40 
120 
40 
0 

0 

0 
0 
60 
160 

0 
60 
180 
0 

40 
0 
0 
0 

0 
0 
0 
400 
0 

0 
0 
0 
0 

0 
80 
0 

0 
0 
0 
0 

800 
40 
40 
1,640 
80 

900 
220 
400 
320 

0 
0 
100 
1,200 
100 

0 
0 
0 
0 

0 
60 
80 
60 

0 

0 
0 
0 
80 

0 
60 
0 
0 

40 
0 
0 
0 

120 
0 
0 
0 
80 

0 
500 
0 
0 

20 
40 
154 

24 
80 
0 
0 

160 
40 
40 
0 
0 

100 
20 
0 
0 

0 
200 
0 
0 
0 

0 
0 
0 
300 

120 
120 
0 
0 

0 
0 
0 
0 
0 

0 
0 
0 
0 

0 
0 
400 
0 

60 
0 
100 
0 
240 

180 
60 
80 
80 

1,220 
7,720 
7,738 

648 
2,160 
6,000 
3,285 

1,124 
3,360 
3,280 
1,600 
2,020 

1,860 
980 
3,100 
20,160 

10,800 
13,600 
213,900 
5,360 
4,720 

24,000 
4,400 
3,300 
10,900 

5,800 
5,320 
2,680 
1,440 

3,440 
840 
6,580 
8,420 
5,320 

-  4,320 
2,420 
5,920 
12,100 

2,740 
1,240 
1,140 
3,860 

4,360 
15,300 
7,500 
25,680 
1,900 

680 
1,560 
320 
340 

544,201,080 
202,136,180 
180,295,247 

619,030,830 
297,341,760 
653,122,000 
115,514,812 

21,790,800 
96,590,840 
118,382,400 
127,930,360 
53,463,040 

42,470,860 
90,934,320 
927,956,220 
3,872,537,280 

3,200,166,960 
4,467,165,760 
2,148,960,400 
190,671,160 
252,704,250 

259,129,000 
1,149,333,480 
641,056,900 
612,686,240 

257,668,840 
223,936,060 
1,578,635,720 
281,604,120 

369,169,240 
3,084,000,880 
583,940,376 
544,041,260 
797,312,600 

488,146,040 
676,773,060 
330,837,120 
511,099,300 

264,225,400 
126,398,510 
182,891,160 
218,768,810 

209,418,675 
277,953,180 
407,573,600 
466,411,780 
364,032,900 

529,250,270 
1,715,442,415 
848,243,820 
387,414,000 

1,042,720 
288,340 
195,484 

1,381,960 
651,200 
1,091,920 
715,697 

1,809,688 
542,680 
453,100 
484,760 
444,900 

363,980 
1,432,400 
2,134,800 
16,092,840 

898,919,800 
42,826,200 
31,091,900 
4,969,580 
2,772,200 

9,695,000 
10,959,400 
15,159,600 
7,796,700 

495,337,320 
4,135,160 
1,717,240 
907,240 

5,833,440 
69,874,760 
1,240,920 
1,519,140 
597,880 

60,463,480 
1,712,720 
32,466,660 
3,201,200 

2,035.720 
1,495,880 
1,967,020 
2,453,060 

1,189,380 
1,281,220 
'732,300 
1,109,040 
799,980 

916,780 
1,757,440 
682,440 
548,700 

545,243,800 
202,424,520 
180,490,731 

620,412,790 
279,992,960 
654,213,920 
116,230,509 

23,600,488 
97,133,520 
118,835,500 
128,451,120 
53,907,940 

42,834,834 
102,366,720 
930,091,020 
3,848,630,120 

4,099,086,760 
4,509,991,960 
2.180,052,300 
195,640,740 
255,476,450 

268,824,000 
1,160,292,880 
656,216,500 
620,482,940 

753,000,160 
228,071,220 
1,580,352,960 
282,511,360 

375,002,680 
3,153,875,640 
585,181,296 
545,560,400 
797,910,480 

548,609,520 
678,485,780 
363,303,780 
514,300,500 

266,261,120 
127,894,390 
184,858,180 
221,221,870 

210,608,055 
279,234,400 
408,305,900 
467,520,820 
364,832,880 

530,167,050 
1,717,199,855 
848,926,260 
387,926,700 

"  21  

"  25 

Feb   3  .  . 

8  

"   15  . 

"  22.  . 

Mar   1 

8.  . 

"  15  

"  22 

"  29  . 

Apr   5 

"   12 

19.  . 

"  26.  ... 

May  3  .  . 

"  10.  .  . 

"  17  

"  24 

"  31.  . 

"   14  . 

"  21  .. 

"  28  

July-  5.  . 

"   12.. 

"   19  

"  26  

Aug   2  .  .  .  . 

9  

"   16 

"  23.  . 

"  30.  . 

Sept.  6  
"  13.  . 

"  20  

"  27  

Oct   4.  . 

"  11  

"  18 

"  25.  . 

Nov   1 

8.  . 

"  15.  . 

"  22  
"  29.. 

Dec.  6  

"  13 

"  20.  . 

"  27  

Average  

37      135 

52 

9,393   723,283,871    34,226,468   756,548,801 

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'00.     Zur  Kenntnis  der  Planktonverhaltnisse  einiger  Gewasser  der 
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'93.     The  Rotifera  of  China.     Journ.  R.  Micr.  Soc.,  1893,  pp.  145- 

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354 


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XXIII.,  pp.  625-627. 
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Government  of  Saratoff.     Bull.  Soc.  Naturalists  of  Moscow,  1903, 

pp.  1-148,  PI.  I.,  II.     (Russian.) 


EXPLANATION  OF  PLATES. 


PLATE  I. 

Seasonal  distribution  of  synthetic  groups  of  planktonts,  Chlorophycece, 
Bacillariacea,  and  Mastigophora,  from  July  1,  1895,  to  October  171896.  Note 
changes  of  scale  indicated  at  bottom  of  diagram.  Numbers  in  column  at  left  apply 
only  to  1895.  In  this  plate  and  in  II.  and  IV.,  apices  exceeding  the  limit  of  the 
diagram  are  dropped  down  between  dotted  lines  to  show  location.  Circles  at 
bottom  indicate  location  of  day  of  full  moon. 

PLATE  II. 

The  same  as  above,  from  July  1,  1897,  to  April  1,  1899.  Note  change  in 
scale  from  previous  plate. 

PLATE  III. 

Seasonal  distribution  of  total  Rotifera  and  Crustacea  from  July  1,  1895,  to 
October  1,  1896.  The  Crustacea  included,  belong  almost  exclusively  to  the  Ento- 
mostraca.  Apices  exceeding  the  limits  of  the  diagram  are  dropped  down  between 
dotted  lines  to  show  location.  Totals  include  both  adult  and  immature  stages  of 
the  Entomostraca  when  detached  from  parent,  and  both  free  and  attached  eggs  of 
the  Rotifera. 

PLATE  IV. 
The  same  as  above,  from  July  1,  1897,  to  April  1,  1899. 

PLATE  V. 

Seasonal  distribution  of  Polyarthra  platyptera.  Total  number  of  individuals, 
not  including  eggs,  represented  by  ordinants,  parts  of  which  exceeding  200,000 
are  represented  by  diagonal  lines  instead  of  solid  vertical  lines.  Thus  parts  of  a 
seasonal  plot  which  overlap  those  above  it  on  the  plate  are  represented  by  the 
diagonally-lined  ordinants. 


(24)  355 


356 


357 


358 


359 


360 


ERRATA  AND  ADDENDA. 


Page  58,  line  7,  for  ovalis  read  ovata. 

Page  85,  line  8,  for  longicaudus  read  longicauda,  and  just  above  Phacus  pleuro- 
nectes  read  the  following  paragraph: — 

Phacus  longicauda  var.  torta,  n.  var. — This  variety,  for  which  I  propose  the 
name  torta  because  of  the  twisted  body,  is  figured  by  Stein  ( '78,  Taf7207 Fig.  3).  It 
occurred  sparingly  in  midsummer  from  July  to  September,  rarely  in  October,  in 
.1896  and  1897. 

Page  91,  line  18,  after  T.  caudata  Ehrb.  read  T.  lagenella  Stein. 

Pages]153,  line  3  from  bottom,  168,  line  16,  and  178,  line  14,  for  '98  read  '98a. 

Pages  156,  line  11,  159,  line  16,  and  161,  line  5  from  bottom,  for  '93  read  '98a. 


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